Add ContentEditable binding for use as `data-editable`.

Adds content-editable two way binding via `data-editable` with keypath. Here's the bucket list of things I know this PR will need off the bat:

## Needs
- [ ] Tests! Some help here would be great as I'm not familiar with JS testing (yet =)
- [ ] Less copy-paste in the bind method (wasn't able to make the adjustments I needed with inheritance)
- [ ] Automatically recognize from standard data-bind, e.g. in `view/dom/readers#bind`, a quick check for `node.contentEditable? and node.contentEditable`
- [ ] Remove the explicit `jQuery` call. Batman needs a platform agnostic set/get innerText DOM helper in `platforms/`
- [ ] Bind as HTML content or text content.

## Wants
- [ ] Two-way filters/plugin mechanism, e.g. for going from a Markdown JS source to DOM, and vice versa.

Any and all suggestions, code comments, and help knocking/adding to the checklist would be super!

Addresses batmanjs/batman#295
~ Jonathan