nice! this is really cool. I hate writing markdown and although I write javascript, html, css, and all sorts of json/regex/xml I can't for the life of me remember markdown (other than bold/links) etc haha.
either way my 1 change would be show the toolbar by default, upon loading my repo for the first time it was a bit un-intuitive but once that appeared it was great. gh should have this built in or switch to html for readmes.