lib/markdown: introduce extended mode

Allow to distinguish standard markdown features from extended one.

Already moved some features from standard to extended mode like:

* end paragraph block with list or code blocks
* underscore in the middle of words are ignored
* fences are recognized

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
lib/markdown: add super and strike to extended mode.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
lib/markdown: add fence meta delcaration to ext mode.

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