allow conjure cli to include sub commands for language specific generators

[qinfchen]

Per Dan's comment on conjure cli

I think the naming looks good here. Long term, I think it would be kinda cool if people could just brew install conjure and it would pull in this core YML -> IR cli, and a few stock generators. I'd imaging people could then run conjure typescript and it would discover the conjure-typescript executable from the $PATH, maybe even prompt people through some steps (where is your input yml file, where would you like to generate your typescript).

example usage:

conjure compile <input> <output>            // compiles input yml files into ir
conjure typescript...                                    // runs typescript generator
conjure java..                                                // runs java generentor

[ferozco]

We can trivially implement this if we rely on the executable existing on their class path. I think the real challenge is packaging and publish the generators in a way that makes it easy to do that. Something like brew on mac or apt on debian