Added method for programatically setting a shortcut which is tied to a NSUserDefaults key

Previously if you had a MASShortcutView, you couldn't programatically set it to some default value. For example on your app's first launch you might have wanted to initialize the global shortcut for a particular feature to some sane default, rather than leaving it empty, but still allowing the user to modify it using a typical MASShortcutView bound to a userDefaultsKey. Now you can.