Enable dirname and filename completion for more flags

These flags are:

 * --source (-s), --cacheDir, --destination (-d) and --logFile
 * --completionfile and --dir.

commands/genautocomplete.go

@@ -48,4 +48,6 @@ func init() {
 	genautocompleteCmd.PersistentFlags().StringVarP(&autocompleteTarget, "completionfile", "", "/etc/bash_completion.d/hugo.sh", "Autocompletion file")
 	genautocompleteCmd.PersistentFlags().StringVarP(&autocompleteType, "type", "", "bash", "Autocompletion type (currently only bash supported)")
 
+	// For bash-completion
+	genautocompleteCmd.PersistentFlags().SetAnnotation("completionfile", cobra.BashCompFilenameExt, []string{})
 }

commands/gendoc.go

@@ -60,4 +60,7 @@ for rendering in Hugo.`,
 
 func init() {
 	gendocCmd.PersistentFlags().StringVar(&gendocdir, "dir", "/tmp/hugodoc/", "the directory to write the doc.")
+
+	// For bash-completion
+	gendocCmd.PersistentFlags().SetAnnotation("dir", cobra.BashCompSubdirsInDir, []string{})
 }

commands/genman.go

@@ -42,4 +42,7 @@ in the "man" directory under the current directory.`,
 
 func init() {
 	genmanCmd.PersistentFlags().StringVar(&genmandir, "dir", "man/", "the directory to write the man pages.")
+
+	// For bash-completion
+	genmanCmd.PersistentFlags().SetAnnotation("dir", cobra.BashCompSubdirsInDir, []string{})
 }

commands/hugo.go

@@ -123,10 +123,14 @@ func init() {
 	HugoCmd.Flags().BoolVarP(&NoTimes, "noTimes", "", false, "Don't sync modification time of files")
 	hugoCmdV = HugoCmd
 
-	// for Bash autocomplete
+	// For bash-completion
 	validConfigFilenames := []string{"json", "js", "yaml", "yml", "toml", "tml"}
 	HugoCmd.PersistentFlags().SetAnnotation("config", cobra.BashCompFilenameExt, validConfigFilenames)
+	HugoCmd.PersistentFlags().SetAnnotation("source", cobra.BashCompSubdirsInDir, []string{})
+	HugoCmd.PersistentFlags().SetAnnotation("cacheDir", cobra.BashCompSubdirsInDir, []string{})
+	HugoCmd.PersistentFlags().SetAnnotation("destination", cobra.BashCompSubdirsInDir, []string{})
 	HugoCmd.PersistentFlags().SetAnnotation("theme", cobra.BashCompSubdirsInDir, []string{"themes"})
+	HugoCmd.PersistentFlags().SetAnnotation("logFile", cobra.BashCompFilenameExt, []string{})
 
 	// This message will be shown to Windows users if Hugo is opened from explorer.exe
 	cobra.MousetrapHelpText = `