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!

Welp, I know I pointed you to that article but apparently https://www.npmjs.org/package/escape-string-regexp is a better one. Mind switching?

Sorry for making you do this two times :(