Setting default templating extension and javascripts/coffeescript option from an initializer.

While developing a project writing --javascript for each generator and renaming each template extension to hbs feels really heavy sometimes. Yet the current --javascript option is enough for many cases adding a config option which can be set in an initializer will speed things up a lot.

Tests are missing for the new features i have added because i don't know how to test options set in initializers. Yet the current tests ran without a failure in my case.

Options can be set in an initializer like below:

```ruby
BackboneOnRails::Config.use_cofeescript = false
BackboneOnRails::Config.jst = "hbs"
````