Add generate --list

Adds `ember generate --list`:

```sh
$ ember generate --list
version: 0.0.32
my-app:
  bar
  foo
ember-cli:
  adapter
  api-stub
  app
  blueprint
  component
  controller
  helper
  initializer
  mixin
  model
  resource
  route
  serializer
  service
  template
  transform
  util
  view
```