Perf: Stop editor parsing code from running multiple times

[jcarpenter]

See markdown.js. It's run multiple times on load.

• First the whole doc parses, and each line is run twice.
• Then the whole doc parses again, and each line is run once.
• Then the whole doc parses again, and each line is run once.

That's four parses, on loading the document.

Might have something to do with the mode overlays, and/or the mode switching?