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.go

@@ -0,0 +1,47 @@
+// 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"
+
+	"github.com/gohugoio/hugo/helpers"
+	"github.com/gohugoio/hugo/transform"
+)
+
+// New creates a function that can be used
+// to inject a script tag for the livereload JavaScript in a HTML document.
+func New(port int) transform.Transformer {
+	return func(ft transform.FromTo) error {
+		b := ft.From().Bytes()
+		endBodyTag := "</body>"
+		match := []byte(endBodyTag)
+		replaceTemplate := `<script data-no-instant>document.write('<script src="/livereload.js?port=%d&mindelay=10"></' + 'script>')</script>%s`
+		replace := []byte(fmt.Sprintf(replaceTemplate, port, endBodyTag))
+
+		newcontent := bytes.Replace(b, match, replace, 1)
+		if len(newcontent) == len(b) {
+			endBodyTag = "</BODY>"
+			replace := []byte(fmt.Sprintf(replaceTemplate, port, endBodyTag))
+			match := []byte(endBodyTag)
+			newcontent = bytes.Replace(b, match, replace, 1)
+		}
+
+		if _, err := ft.To().Write(newcontent); err != nil {
+			helpers.DistinctWarnLog.Println("Failed to inject LiveReload script:", err)
+		}
+		return nil
+	}
+}