Add `grunt-broccoli-build` plugin to list

This is a much simpler and straight-forward Grunt plugin than the `grunt-broccoli` plugin, because it only does one thing — runs a Broccoli build — uses a peer dependency on Broccoli, and has less dependencies.
Merge pull request #68 from ericf/grunt-broccoli-build-plugin

Add `grunt-broccoli-build` plugin to list