Fix bugs in Markdown mode and test harness (closes #1622)

I'm kind of surprised this even worked before. Shouldn't consume characters `stream.match()` (`inlineElement()` call takes care of that). Also, while fixing it, I noticed `<...>` was turning into an HTML tag instead of displaying. Added as a separate commit in case you want to adjust it before committing. I'm fairly confident you only need to escape `<` and `&` (see [here](http://www.whatwg.org/specs/web-apps/current-work/multipage/tokenization.html#data-state)).