Enable dirname and filename completion for more flags

These flags are:

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

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{})
 }