Markdown

# lib: Introduce `markdown`, a Markdown parser for Nit.

_Please consider only f9c54c4, since previous commit are merged from #627._

This is the first step towards a better markdown interpretation into Nit code comments.
Next step will be to replace the current markdown parser used in `src/markdown`.

This parser try to be a little more closer to the Markdown specification from http://daringfireball.net/projects/markdown/ than the actual parser.
That said, some rules are not strictly followed as explained in the `README`.
Some other rules or features are not yet fully implemented, see `TODO` and `FIXME` in code.