Addon API for blueprint packages.

Adds an addon api for blueprint packages.
    
A blueprint addon must:
* specify the keyword `ember-cli-blueprint` in `package.json`
* contain a blueprints folder

Everything else is automagical. Just npm install the blueprint package and use it's blueprints.
Then, we might even want to change the addon API from `blueprintsPath` to `blueprintLookupPath`?