improve ember generate help + documentation

Starts to address #43 by depending on loom and loom-generators-ember-appkit, which provides the ability to run commands like `ember generate controller foo type:array` 

- [x] Better test coverage
- [ ] `ember help generate` should give a list of available generators (not sure if loom has this ability currently)
- [x] `ember g ...` should be an alias for `ember generate ...`
- [ ] Document `ember generate init` and how to override generators/create your own