Merge pull request #459 from pavelbinar/v0.5.0

move plugin examples to `examples/plugins`
converting the assemble.context object to a method to be used in helpers
Merge branch 'v0.5.0' of https://github.com/assemble/assemble into v0.5.0
integrating the defaults.yml file
updating dovetail and plugins
updating how helpers are loaded to use boson
adding mixins to lodash with option to use  and add your own prefix
adding plugin loader test to make sure plugins get loaded
adding better namespacing for lodash mixins and tests