Add support for minification of final output

Hugo Pipes added minification support for resources fetched via ´resources.Get` and similar.

This also adds support for minification of the final output for supported output formats: HTML, XML, SVG, CSS, JavaScript, JSON.

To enable, run Hugo with the `--minify` flag:

```bash
hugo --minify
```

This commit is also a major spring cleaning of the `transform` package to allow the new minification step fit into that processing chain.

Fixes #1251

publisher/publisher.go

@@ -0,0 +1,160 @@
+// Copyright 2018 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package publisher
+
+import (
+	"errors"
+	"io"
+	"sync/atomic"
+
+	"github.com/gohugoio/hugo/media"
+
+	"github.com/gohugoio/hugo/minifiers"
+
+	bp "github.com/gohugoio/hugo/bufferpool"
+	"github.com/gohugoio/hugo/helpers"
+
+	"github.com/spf13/afero"
+
+	"github.com/gohugoio/hugo/output"
+	"github.com/gohugoio/hugo/transform"
+	"github.com/gohugoio/hugo/transform/livereloadinject"
+	"github.com/gohugoio/hugo/transform/metainject"
+	"github.com/gohugoio/hugo/transform/urlreplacers"
+)
+
+// Descriptor describes the needed publishing chain for an item.
+type Descriptor struct {
+	// The content to publish.
+	Src io.Reader
+
+	// The OutputFormat of the this content.
+	OutputFormat output.Format
+
+	// Where to publish this content. This is a filesystem-relative path.
+	TargetPath string
+
+	// Counter for the end build summary.
+	StatCounter *uint64
+
+	// Configuration that trigger pre-processing.
+	// LiveReload script will be injected if this is > 0
+	LiveReloadPort int
+
+	// Enable to inject the Hugo generated tag in the header. Is currently only
+	// injected on the home page for HTML type of output formats.
+	AddHugoGeneratorTag bool
+
+	// If set, will replace all relative URLs with this one.
+	AbsURLPath string
+
+	// Enable to minify the output using the OutputFormat defined above to
+	// pick the correct minifier configuration.
+	Minify bool
+}
+
+// DestinationPublisher is the default and currently only publisher in Hugo. This
+// publisher prepares and publishes an item to the defined destination, e.g. /public.
+type DestinationPublisher struct {
+	fs     afero.Fs
+	minify bool
+	min    minifiers.Client
+}
+
+func NewDestinationPublisher(fs afero.Fs, outputFormats output.Formats, mediaTypes media.Types, minify bool) DestinationPublisher {
+	pub := DestinationPublisher{fs: fs}
+	if minify {
+		pub.min = minifiers.New(mediaTypes, outputFormats)
+		pub.minify = true
+	}
+	return pub
+}
+
+// Publish applies any relevant transformations and writes the file
+// to its destination, e.g. /public.
+func (p DestinationPublisher) Publish(d Descriptor) error {
+	if d.TargetPath == "" {
+		return errors.New("must provide a TargetPath")
+	}
+
+	src := d.Src
+
+	transformers := p.createTransformerChain(d)
+
+	if len(transformers) != 0 {
+		b := bp.GetBuffer()
+		defer bp.PutBuffer(b)
+
+		if err := transformers.Apply(b, d.Src); err != nil {
+			return err
+		}
+
+		// This is now what we write to disk.
+		src = b
+	}
+
+	f, err := helpers.OpenFileForWriting(p.fs, d.TargetPath)
+	if err != nil {
+		return err
+	}
+
+	_, err = io.Copy(f, src)
+	if err == nil && d.StatCounter != nil {
+		atomic.AddUint64(d.StatCounter, uint64(1))
+	}
+	return err
+}
+
+// Publisher publishes a result file.
+type Publisher interface {
+	Publish(d Descriptor) error
+}
+
+// XML transformer := transform.New(urlreplacers.NewAbsURLInXMLTransformer(path))
+func (p DestinationPublisher) createTransformerChain(f Descriptor) transform.Chain {
+	transformers := transform.NewEmpty()
+
+	isHTML := f.OutputFormat.IsHTML
+
+	if f.AbsURLPath != "" {
+		if isHTML {
+			transformers = append(transformers, urlreplacers.NewAbsURLTransformer(f.AbsURLPath))
+		} else {
+			// Assume XML.
+			transformers = append(transformers, urlreplacers.NewAbsURLInXMLTransformer(f.AbsURLPath))
+		}
+	}
+
+	if isHTML {
+		if f.LiveReloadPort > 0 {
+			transformers = append(transformers, livereloadinject.New(f.LiveReloadPort))
+		}
+
+		// This is only injected on the home page.
+		if f.AddHugoGeneratorTag {
+			transformers = append(transformers, metainject.HugoGenerator)
+		}
+
+	}
+
+	if p.minify {
+		minifyTransformer := p.min.Transformer(f.OutputFormat.MediaType)
+		if minifyTransformer != nil {
+			transformers = append(transformers, minifyTransformer)
+		}
+	}
+
+	return transformers
+
+}

publisher/publisher_test.go

@@ -0,0 +1,14 @@
+// Copyright 2018 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package publisher