Support for filepicker.io file uploads

Sam and eviltrout. I tried to isolate the code to make it as easy to parse the additions as possible.  I'm not quite sure about how to properly add settings to the project, so everything is currently stored in javascript variables with names prefixed as "filepicker_" in the Markdown.Editor.js.

You can see how this works by clicking on the image button on the toolbar in
http://discourse.mcneel.com/t/no-spell-check-no-image-attachment-16000-character-limit-this-is-progress