Provide a dropdown for switching between GFM and regular Markdown rendering.
(Previously the preview used GFM-style linebreak handling but otherwise
acted like regular Markdown).
Open links in browser when clicked - using a similar approach to the main
Brackets UI, except directed at the preview's iframe instead
Merge pull request #33 from peterflynn/render-settings

Dropdown for enabling GFM rendering behavior; and make links clickable