Adds an 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.
I would prefer just to call `project.initializeAddons()` here, and provide the `addonBlueprintPaths` array to `Blueprint.lookup` and `Blueprint.list` via the `paths` option.