Add "--add-comments" option

Hi,

I'd love to have a "--add-comments" option, which is available for original `xgettext` in GNU gettext utilities. The option allows developers to use not only "l10n:" tag but also any tag names.

Thanks!

Two things here: 
  1. You need to RegEx escape the provided tag since it is not guaranteed to be regex safe. You may use something like this: http://stackoverflow.com/a/11177116/90297
  2. Since we are "building" the RegEx manually, something like the following might be more readable:

        var commentRegex = new RegExp([
            "^\\s*" + tagName,  // The "TAG" provided externally
            "^\\/", // The "///" style comments which is the xgettext standard
        ].join("|"));