Remove dependency on ember-cli-sass and sass

[bertdeblock]

• Uses a pre-commit hook to ensure compiled styles are automatically included in every commit
• Adds the ability to exclude Ember Freestyle's styles in case people want to define their own styles or if they want to import Ember Freestyle's styles explicitly (when they use ember-cli-sass and want to override variables).

Closes #143, closes #152, closes #172 and closes #537.

[lukemelia]

Great solution!

[lukemelia]

@bertdeblock I think this change requires some action on the part of current users to avoid including the styles twice, correct?

[bertdeblock]

@lukemelia afaict, everything should remain the same for current users. The styles in the app folder won't end up in the app's build unless explicitly imported via @import 'ember-freestyle'. Though, maybe this change is drastic enough to mark it as breaking nonetheless, just to be sure.