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

transform/livereloadinject/livereloadinject_test.go

@@ -0,0 +1,41 @@
+// 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 livereloadinject
+
+import (
+	"bytes"
+	"fmt"
+	"strings"
+	"testing"
+
+	"github.com/gohugoio/hugo/transform"
+)
+
+func TestLiveReloadInject(t *testing.T) {
+	doTestLiveReloadInject(t, "</body>")
+	doTestLiveReloadInject(t, "</BODY>")
+}
+
+func doTestLiveReloadInject(t *testing.T, bodyEndTag string) {
+	out := new(bytes.Buffer)
+	in := strings.NewReader(bodyEndTag)
+
+	tr := transform.New(New(1313))
+	tr.Apply(out, in)
+
+	expected := fmt.Sprintf(`<script data-no-instant>document.write('<script src="/livereload.js?port=1313&mindelay=10"></' + 'script>')</script>%s`, bodyEndTag)
+	if string(out.Bytes()) != expected {
+		t.Errorf("Expected %s got %s", expected, string(out.Bytes()))
+	}
+}