[Feature request] Support for GitHub Flavored Markdown

mat

Hello,

I wanted to pitch the idea of allowing users to switch between different Markdown specs, namely GitHub Flavored Markdown.

In particular, I wanted to use GFM style strikethroughs (GFM is a superset of CommonMark, which is a popular spec of the original Markdown by John Gruber) but after doing a bit of research, I learned that strikethrough is not defined in the original Markdown spec and can be a controversial topic among Markdown-nerds. It is included in the GFM spec as an extension, so instead of requesting for strikethrough text support, I figure it might be more sensible to ask if switching to different Markdown styles is the better request.