Add ProseMirror example

[keybits]

I'd love an example of how to make InlineAttachment work with ProseMirror.

Specifically I'd like to see how to make it work with the 'Markdown' editing mode of this ProseMirror example: http://prosemirror.net/demo_markdown.html

(Many thanks for your work on this - very useful, sorry my JS isn't good enough to work out the best way to make it work with ProseMirror)