Dropdown for enabling GFM rendering behavior; and make links clickable

The current version uses GFM-style linebreak handling but otherwise renders like regular Markdown.  This adds UI to explicitly toggle between full GFM or no GFM at all.  I set the dropdown picker to only appear when you hover over the iframe, to reduce clutter... but that could be changed if need be.

This also changes the behavior of links so that `openURLInDefaultBrowser()` is called to when they're clicked.