Add an "highlightFormatting" option

[SamyPesse]

The markdown mode has an option to highlight formatting.

For example, instead of parsing hello **world** as ["hello ", "**world**"], it'll parse it as ["hello ", "**", "world", "**"].

[nerk]

🎅 🎄 Since parsing is almost completely done with regular expressions, I fear we would basically need to rewrite most of this to support such a feature. Correct me if I'm wrong...