Add lingua plugin

This pull requests adds a lingua plugin to Mako. In addition it adds a couple of high-level tests for the existing Babel plugin, and refactors that plugin a bit to allow reuse of the basic parsing code.

Please note that this is still a work in progress, so please don't merge it as-is. There are a couple of things I still want to do:

• Run this on some real world mako templates. Since I've never used Mako myself any pointers to examples would be very welcome :)

• Lingua itself needs a better API to access its Python parser. I want to have that in place before getting this out - the current use of _extract_python is a bit dirty.