lib: Introduce `markdown`, a Markdown parser for Nit.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
tests: rename `test_markdown.nit` to `test_docdown` to avoid name conflicts with `lib/markdown`.

`src/markdown` will be replaced by `src/doc_down` in a later PR.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
tests: update tests for markdown.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
Merge: 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.

Pull-Request: #680
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Merge branch 'master' of https://github.com/privat/nit