Add Bash completion

Add a new command, genautocomplete, wich generates a Bash completion script (zsh and others later).

The script is by default written to `/etc/bash_completion.d/hugo.sh`; this can be set in `--completionfile=/some/file`.

Fixes #438

commands/hugo.go

@@ -76,6 +76,7 @@ func AddCommands() {
 	HugoCmd.AddCommand(newCmd)
 	HugoCmd.AddCommand(listCmd)
 	HugoCmd.AddCommand(undraftCmd)
+	HugoCmd.AddCommand(genautocompleteCmd)
 }
 
 //Initializes flags
@@ -103,6 +104,12 @@ func init() {
 	HugoCmd.Flags().BoolVarP(&NoTimes, "noTimes", "", false, "Don't sync modification time of files")
 	hugoCmdV = HugoCmd
 
+	// for Bash autocomplete
+	validConfigFilenames := []string{"json", "js", "yaml", "yml", "toml", "tml"}
+	annotation := make(map[string][]string)
+	annotation[cobra.BashCompFilenameExt] = validConfigFilenames
+	HugoCmd.PersistentFlags().Lookup("config").Annotations = annotation
+
 	// This message will be shown to Windows users if Hugo is opened from explorer.exe
 	cobra.MousetrapHelpText = `