Return change object with onChange

[jrunning]

At present, the react-code-mirror component only gives the new content of the document to onChange handlers (presumably for consistency with React's textarea implementation). However, CodeMirror's internal change event fires with additional, useful information:

The changeObj is a {from, to, text, removed, origin} object containing information about the changes that occurred as second argument.

It would be great if the React component also provided this information to onChange handlers.

[ForbesLindesay]

Happy to accept pull requests.