Add Jack mode

I tested by manually running your https://github.com/creationix/ace-builds/blob/master/update.sh script on my local ace version and installing into zed.

![jack-ace](https://f.cloud.github.com/assets/89353/1247161/d4e87bdc-2ab7-11e3-9e79-0176f01682fb.png)

Depends on ace with https://github.com/ajaxorg/ace/pull/1636 applied.