Add support for `--add-comments[=tag]`

[Minoru]

xtr extracts all the comments that precede the keyword. xgettext on the other hand lets us narrow it down to comments that start with a specific "tag": https://www.gnu.org/software/gettext/manual/gettext.html#Operation-mode Could xtr support this too?

[ogoffart]

This would make sense, yes.