Add support of custom providers.

We need to realize something like this:

>>> from elizabeth import Generic

>>> generic = Generic('en')

>>> class SomeProvider():
        def hello(self):
            return "Hello!"

>>> class Another():
        def bye(self):
            return "Bye!"

>>> generic.add_provider(SomeProvider)
>>> generic.add_provider(Another)

>>> generic.someprovider.hello()
>>> generic.another.bye()
# Hello!
# Bye!

[Battleroid]

Sounds neat. Is there anything in particular you would like me to do? I’ll try to get started on it in the morning.

[Battleroid]

No problem, glad it works.