StackEdit[0] pretty much perfected what I needed out of a markdown editor - I just need somewhere to write my tickets/docs that wasn't Github so that I could format it properly while writing. I still use it from time to time
No I don't believe it supports collaboration, though I could be wrong. This is really a pretty notepad with great Markdown integration, nothing much more (which is why it's still good!)
[0]: https://stackedit.io/