🔗 Add support for github issue previews

[rowanc1]

• See 🔗 Add support for github issue previews mystmd#389

[rowanc1]

cc @choldgraf this is the work for the previews we were talking about! I think there is a bunch to improve (I have just skipped over github markdown parsing for the body text). That choice is mainly to keep the JS bundle small, and I think for a preview, it mostly looks ok.

I have picked up on the tags, and have a simple choice to show white or black text on it.

Going to push through and get this out the door and start testing it more! 🚀

@tavin if you have thoughts, always feel free to comment on the PRs and I/we will follow up and fix things (defaulting to ship/show pattern, to move fast, but absolutely open to feedback and fixing things even though a PR is merged!!).