Display message to user if an ember-cli update is available (and add `ember update` command)

This is a first pass at checking for ember-cli updates and displaying a message to the user if an update is available (fixes https://github.com/stefanpenner/ember-cli/issues/335).

### TODOs
- [x] Persistence via .ember-cli
- [x] Option to disable via .ember-cli
- [x] `ember update` command
- [x] Tests