Also merge "-lawgiver.pot"-files, adding ftw.lawgiver support.

`ftw.lawgiver` will generate `ftw.lawgiver-lawgiver.pot` files for merging into the regulare pot file.
This pull-requests adds support for merging those.

It is not possible to use `-content.pot` and `-lawgiver.pot` at the same time because `i18ndude` only supports for two merge files (and `-manual.pot` is always first), but that should not be a problem since those are usually on different domains.

@deiferni can you take a look?