add support for plugins running once, before and after
Merge branch 'feature-plugins-once' of https://github.com/adjohnson916/assemble into plugins
Implementing multiple plugin stages.

Expanding on the before/after/each model of plugins to have
multiple stages, mostly pre and post each assemble step.

Updated the tests to work with the new stages.
Merge pull request #370 from doowb/plugins

Plugins