Added Ace editor multiple cursors functionality and example

Hi, this is my first attempt on multiple cursors for ace editors. Feedback is more than welcome.

One problem I have is that `TogetherJSConfig_dontShowClicks` doesn't work for an ace editor, see the example here: https://github.com/triglian/togetherjs/blob/develop/site/examples/acecursors/index.html#L36