fix ctrl-click not working on chrome. Fixes #859

The codemirror internal on("mousedown", ...) is not actually
calling a handler (see https://github.com/marijnh/CodeMirror/issues/1247#issuecomment-13600740),
I took the approach mentioned there to get the event when bubbled out.
Merge pull request #884 from herby/fix-ctrl-click

fix ctrl-click not working on chrome