Enable inline editing if contenteditable is true.

Since inline editing requires the contenteditable attribute to be set to
true, this was an easy way to have both inline editing and the
traditional editor live side by side.

Note that the check for contenteditable ideally should be done with
isContentEditable, but it didn't work correctly in Chrome, probably due
to this ticket:
https://code.google.com/p/chromium/issues/detail?id=313082

Fixes #1