Every time I need to edit a *.md file in GitHub I'm forced to push 50 commits until I get the desired result or use one of the existing tools that don't allow live-preview (at least none of the ones I found searching in Google).
After 15 min I came up with a solution based on markdown.js. Maybe it's not the best looking tool, nor has any options at all, but it does the job quite well. Enjoy!
PS: You might want to run that in full screen.