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

minifiers/minifiers_test.go

@@ -0,0 +1,35 @@
+// 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 minifiers
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+
+	"github.com/gohugoio/hugo/media"
+
+	"github.com/gohugoio/hugo/output"
+	"github.com/stretchr/testify/require"
+)
+
+func TestNew(t *testing.T) {
+	assert := require.New(t)
+	m := New(media.DefaultTypes, output.DefaultFormats)
+
+	var b bytes.Buffer
+
+	assert.NoError(m.Minify(media.CSSType, &b, strings.NewReader("body { color: blue; }")))
+	assert.Equal("body{color:blue}", b.String())
+}