Add Mustache Render Context

This context uses the mustache templating system to write out files. Language
defnitions are then made in `.mustache` files and a simple wrapper class is used
to render them.

This is still a bit messy as there is quite a bit of logic in the views (both
callback and mustache). It would be better if some of this computation could
be moved into the grammar class.
Add TODO Items

Add a file containing TODO items for the project. The plan is to put thoughts
in here and then move them to issues when I feel like working on them. This
means that short-lived TODOs don't need an issue. It also means that others can
see what I plan for the project and chip in if they feel like it.