Initial commit

Support for Hack syntax and hh_client typechecker integration.  Plugins
were developed based on the pre-release integrations originally found
at:

  https://github.com/facebook/hhvm/tree/master/hphp/hack/editor-plugins/vim

Changes since that version include:
- Removed stray "fb" prefixes and other FB junk.
- Removed broken hack-color.vim plugin.
- Fixed bugs in generic type syntax highlighting.
- Rewrote omnicompletion natively to eliminate Python dependency.
- Made various improvements to the typechecker and omnicompletion.
- Fixed the docs.