Undo markdown

[oscarmorrison]

Most other editors have a function where if you hit (⌘ + Z / Ctrl + Z) it will undo the automarkdown, and leave the text as it was.

Eg. if I type **bold**_ ( _ denoting a space) this changes it to bold, however if I undo, it will just make it **bold**, when I want it to be **bold**_.

TL;DR: allow native undo behavior, to stop the auto markdown.