RFC: Engines

[RFC](https://github.com/tomdale/rfcs/blob/master/active/0000-engines.md)
@stefanpenner and I have talked about this in part. Currently in ember-cli apps and addons have two different compilation pipelines. Part of this effort might include moving ember-cli to consider the application itself as an addon/engine. This would consolidate the compilation pipelines and would demand that the build system revolve around engines.