PERF: Fix slow greedy match in `oneElement`

Having a greedy regexp in `oneElement` had major performance issues in Chrome when parsing very large inline blocks.

This re-write does not change any functionality but improves this performance issue. Even though I'm submitting it, it was written by @SamSaffron although I touched it up a bit :)

All tests pass, as does all of Discourse's tests and MDTest 1.1