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

2 files changed, 16 insertions, 6 deletions

diff --git a/commands/commands.go b/commands/commands.go
index 88939e600..17c9e15cc 100644
--- a/commands/commands.go
+++ b/commands/commands.go
@@ -177,6 +177,7 @@ Complete documentation is available at http://gohugo.io/.`,
 	cc.cmd.Flags().BoolVarP(&cc.buildWatch, "watch", "w", false, "watch filesystem for changes and recreate as needed")
 
 	cc.cmd.Flags().Bool("renderToMemory", false, "render to memory (only useful for benchmark testing)")
+	cc.cmd.Flags().Bool("minify", false, "minify any supported output format (HTML, XML etc.)")
 
 	// Set bash-completion
 	_ = cc.cmd.PersistentFlags().SetAnnotation("logFile", cobra.BashCompFilenameExt, []string{})
diff --git a/commands/hugo.go b/commands/hugo.go
index 312767499..1381bf210 100644
--- a/commands/hugo.go
+++ b/commands/hugo.go
@@ -209,16 +209,25 @@ func initializeFlags(cmd *cobra.Command, cfg config.Provider) {
 		"verboseLog",
 	}
 
+	// Will set a value even if it is the default.
+	flagKeysForced := []string{
+		"minify",
+	}
+
 	for _, key := range persFlagKeys {
-		setValueFromFlag(cmd.PersistentFlags(), key, cfg, "")
+		setValueFromFlag(cmd.PersistentFlags(), key, cfg, "", false)
 	}
 	for _, key := range flagKeys {
-		setValueFromFlag(cmd.Flags(), key, cfg, "")
+		setValueFromFlag(cmd.Flags(), key, cfg, "", false)
+	}
+
+	for _, key := range flagKeysForced {
+		setValueFromFlag(cmd.Flags(), key, cfg, "", true)
 	}
 
 	// Set some "config aliases"
-	setValueFromFlag(cmd.Flags(), "destination", cfg, "publishDir")
-	setValueFromFlag(cmd.Flags(), "i18n-warnings", cfg, "logI18nWarnings")
+	setValueFromFlag(cmd.Flags(), "destination", cfg, "publishDir", false)
+	setValueFromFlag(cmd.Flags(), "i18n-warnings", cfg, "logI18nWarnings", false)
 
 }
 
@@ -229,9 +238,9 @@ var deprecatedFlags = map[string]bool{
 	strings.ToLower("canonifyURLs"):          true,
 }
 
-func setValueFromFlag(flags *flag.FlagSet, key string, cfg config.Provider, targetKey string) {
+func setValueFromFlag(flags *flag.FlagSet, key string, cfg config.Provider, targetKey string, force bool) {
 	key = strings.TrimSpace(key)
-	if flags.Changed(key) {
+	if (force && flags.Lookup(key) != nil) || flags.Changed(key) {
 		if _, deprecated := deprecatedFlags[strings.ToLower(key)]; deprecated {
 			msg := fmt.Sprintf(`Set "%s = true" in your config.toml.
 If you need to set this configuration value from the command line, set it via an OS environment variable: "HUGO_%s=true hugo"`, key, strings.ToUpper(key))