Add remarkable to Parsing

This adds [remarkable](https://github.com/jonschlinkert/remarkable).
Advantages:
- Supports the CommonMark spec + syntax extensions + sugar (URL autolinking, typographer).
- Configurable syntax! It is possible to add new rules and replace existing ones.
- High speed!

Some benchmark results here: https://github.com/twbs/bootstrap/pull/14877#issuecomment-60767688