Suport composing/importing grammars

[byteit101]

One of the advantages of PEG-based grammars is that they are composable.
It would be very nice if canopy supports either composing or importing grammars.

I have a start of this implementation at https://github.com/byteit101/canopy/tree/multi_grammar but I wasn't sure how to expose actions for Java (multiple interfaces?), but otherwise it should "work". Importing should also be doable, but I haven't implemented that yet.