add shortcuts at the save dialog for mac

As we discussed at [here](http://haroopad.userecho.com/topic/248042-mac-commandd/), there are various ways to deal with shortcuts in a dialog box for MacOS. This request is one of the approaches.

  * Command+D - Don't Save
  * Enter Key - Always Save
  * Escape Key - Always Cancel

And actually I'm a big fan of haroopad, thank to @rhiokim and the team, you guys are doing awesome, kudos!