extraction: Babel now supports extraction by filename as well as by dir

extraction: Babel now supports extraction by filename as well as by dir

One can now supply a filename or a directory to be extracted. For
large codebases, this allows the consumer to optimize their
string extraction process by, for instance, only supplying the
files that have actually been changed on the given dev's branch
compared to master.

Relates to https://github.com/python-babel/babel/issues/253 . I
don't want to say "fixes", but makes further optimization
unnecessary for most use cases.