Allow for custom ember app name in generators

Add a `-n` [`--app_name`] option to generators which allows users to specify their Ember app name rather than using the ::Rails.application.class.name default.

For example

```bash
bundle exec rails g ember:bootstrap -n MyApp
```
Would use `MyApp` as the ember app namespace

Also addresses https://github.com/emberjs/ember-rails/issues/174