Allow specifying custom translation macros (e.g. Pl_ instead of N_)

[MichaelChirico]

{data.table} uses Pl_:

https://github.com/Rdatatable/data.table/blob/f6dca2d3247b1ca460b59d1eb13b2ebfeab2e259/src/po.h#L4-L9

This should be supported.

[MichaelChirico]

It should be supported for both check_untranslated_src() and po_extract().