Add Tilt Converter

Add [Tilt](https://github.com/rtomayko/tilt/) converter. Faster, more formats available. Still support custom converters.

- [x] Add Tilt converter
- [ ] Figure out which Tilt-supported templates we can support
- [ ] Move markdown converters to use Tilt wherever possible
- [ ] Add unit tests for Tilt converter
- [ ] Add features for Tilt converter

/cc @benbalter @mattr- @mojombo