[Feature request] GFM support in `@vuepress/markdown`

[Mister-Hope]

Clear and concise description of the problem

I receive many users complaining that they are unable to use our tools to build a existing markdown folder in their project.

Mainly because we do not have full support of GFM.

It's good for us to support "standard" markdown files in GitHub.

Suggested solution

Add a new gfm option

Alternative

use vuepress-plugin-md-enhance with { gfm: true } option

[meteorlxy]

I'm planning to extract those md-it plugins to a standalone project

[Mister-Hope]

Any update with this one?