Add config.ember.app_name option.

Once set (usually in `config/application.rb`), you would no longer need
to specify the `--app-name` argument for each generator that you run.

Closes https://github.com/emberjs/ember-rails/issues/285.
Merge pull request #286 from rjackson/add_app_name_config_option

Add config.ember.app_name option.