rewrite to support new plugin API

hi, this is basically a complete rewrite of this plugin to support the new v2 api; i started with your initial code, but i didn't care about keeping compatibility with v1 so i ended up redoing most of the things.

i would like this version to be the version that's in rubygems for easy `vagrant plugin install` ability. however, since this is such a massive rewrite i would understand if you don't want to merge the changes. in that case, let me know and i'll rename the plugin and create a separate rubygem for it.

i'm also happy to help maintain this project, since it's totally useful for me and for others at Airbnb; feel free to add me as a maintainer on this repo.