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>