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

:construction: 

`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?

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Update:
The concept is not working how I expected it: the problem is once again that `plone` pot files are not built and therefore the `plone-lawgiver.pot` which is generated by `ftw.lawgiver` is never merged..