abstract out a backend api

[willkg]

Currently Dennis code is pretty tied to .po and .pot files. Dennis would be more useful if it could understand other formats.

This issue covers creating a backend API and then rewriting the code so that it uses the backend specified to manipulate files. This then lets us write backends for .po/.pot, the way Plone does .po/.pot files, and other formats that are supported by Pontoon.

[jayvdb]

https://pypi.python.org/pypi/Babel (https://github.com/python-babel/babel) includes support for several message storage backends : https://github.com/python-babel/babel/tree/master/babel/messages .

It also has some validation routines that may be useful
https://github.com/python-babel/babel/blob/master/babel/messages/checkers.py

[willkg]

I'm not sure Babel helps here. Seems like it supports just gettext message catalogs (.po, .pot, .mo). It doesn't support Fluent or other systems. That's what I was thinking might be interesting to add support for.