Add blueprint descriptions to ember help generate

Also display `(overridden)` next to overridden blueprints.

Addresses #1237
Addresses #1274

![ember-cli](https://cloud.githubusercontent.com/assets/34030/4287644/ba22f786-3da4-11e4-89c4-0fe9ee0269c8.png)

**TODO:**

- [x] Add support for `--verbose` flag to show overridden blueprints (default: false)
- [x] Add descriptions to existing blueprints
- [x] Discuss `availableOptions` with regard to blueprints