Add Basic HTML5 Support to Rich Text

Currently, Marketo strips out any HTML5 attributes/elements that are placed in a rich text editor, be it on a landing page or in a token. While I realize that using HTML5 can be problematic in a few instances (especially when there's a lack of a Modernizr.js or equivalent fallback), it would be nice to have the following available:

• HTML5 form attributes
• Download attribute
• Details and summary elements