Add blueprint descriptions to ember help generate

Also display '(overridden)' next to overridden blueprints.

Addresses #1237
Addresses #1274

Pass options through printBasicHelp & printDetailedHelp

Hide overridden blueprints unless verbose flag is set

Add example descriptions to built-in blueprints

Add availableOptions to Blueprint
Merge pull request #1279 from jgwhite/blueprint-list-description-and-overrides

Add blueprint descriptions to ember help generate