[RFC] Allow ember-cli-yuidoc to be mounted as an engine

# Motivation

For application developers, this feature allows docs to be integrated as part of an Ember application. The integration would be straight forward too.

For add-on developers, this feature allows integration with `ember-cli-github-pages`. `ember-cli-github-pages` is another add-on that let add-on developers to publish the add-on dummy app as Github pages. However, this runs in conflict with YUIdoc because YUIdoc is not built as a single page application. This RFC aims to solve the problem by allowing the dummy app to mount to docs as an engine.