Upgrade to MMD4

[dtjm]

Better performance, less memory leaks, new reentrant parser code:
https://github.com/fletcher/MultiMarkdown-4

If anyone is interested in helping with this, please let me know!

[devinrhode2]

👍

[dtjm]

This actually shouldn't be too hard. I just wrote a wrapper for MMD4 in Go, only took an hour or two.