[WIP] Add blueprint descriptions to ember help generate

Also display `(overridden)` next to overridden blueprints.

Addresses #1237
Addresses #1274

![screen shot 2014-07-05 at 18 27 52](https://cloud.githubusercontent.com/assets/34030/3487179/bacff70c-0469-11e4-89f4-5e31f5b06438.png)

TODO:

- [ ] Add support for `--show-overriden` flag to show/hide overridden blueprints (hide by default)
- [ ] Add descriptions to existing blueprints
- [ ] Discuss `availableOptions` with regard to blueprints
- [ ] Discuss output formatting