Fixes a problem with NSTask blocking the main thread.

NSTask seems to block the app while a shell command is running. 
For instance if you run `say` with a paragraph the app will be unresponsive until the speech ends.

Added an option in `shell` to run in background and ignore the output:

bind "S", ["cmd", "alt", "ctrl"], -> shell '/usr/bin/say', [api.selectedText()], null, null, {"donotwait": true}