Allow configuring languages to sync or to ignore

[dwaynebailey]

Currently we are importing the whole of the upstream repo. We're assuming that all languages of interest can and should be edited on Pootle.

This would not be true on some of our installations. In those cases we want to be able to:

1. Specify the languages that we are managing e.g. af, zu
2. Specify the languages we are not importing at all e.g. -fr, -it
3. Allow the specification of special use cases. Those that we import but only as an alt-src language. Thus we import them but don't show them in Pootle.

[phlax]

This shouldnt be too hard - we can add either/both only_langs/exclude_langs to the config