2.0 polishing

[jedfoster]

• [x] HTML
• [x] parse Haml
• [x] parse Markdown
• [x] parse Textile
• [x] save HTML with gist
• [x] persistent layout configurations (save panel positions with localStorage)
• [x] custom layout configurations
• [x] show/hide panels
• [x] move extension list out of control menu, to Sass form header bar
• [x] retrieve HTML from gist
• [x] sandbox HTML rendering in a subdomain
• [x] update HTML panel syntax highlighting when syntax changes
• [x] prettier show/hide panel controls

[jedfoster]

Dropped support for Slim. May be able to revisit it once the sandbox is in place.

[jedfoster]

Calling this pretty much done with 7faeac4. This has dragged on far too long.