The current way to configure compare-locales has a lot of
assumptions that make our l10n system really stubborn.

The generic configuration is independent of python, and uses
toml files for configuration. They're still modular, but
there's only one file format.

See http://moz-l10n-config.readthedocs.io/en/latest/fileformat.html
for the specification.

Also fixes a few nits in filter.py, where we compared the
entity key as bool, which is false if we pass in ''.
Explicitly compare as "entity is None" to be precise about
when we're checking files.

MozReview-Commit-ID: 5TmfobaImF4

--HG--
extra : rebase_source : 84e56eb2076e74f79677df9e0368811579c1f173