Add code formatting integration to Elixir cells

[jonatanklosko]

Closes #296.

I figured out a way that should overcome the concerns I had with formatting and is actually pretty simple. There is a detailed comment in the code, but the important parts are:

• user originates the formatting and it happens within editor as a regular edit (consequently it's a normal operation on the undo stack)
• the formatting operation isn't a replace-all, but a set of operations that introduce the minimal diff, consequently the Delta sent from the client is reasonable, which should make conflict resolution work reliably if needed
• formatting is only applied if the editor is intact, any change (remote or local) cancels the formatting, and that eliminates a whole category of conflicts

editor_formatting.mp4

[jonatanklosko]

@josevalim please verify that this works as expected for you and if you can undo the formatting edit :)

[benjreinhart]

@jonatanklosko it works for me on Mac