Refactor --icon to allow image,icon, or extname

Allow the icon switch to be an image, icon, or extension. This would be much more usable than --icon, --image, and --iconpath

So for instance the following would all be legal

# Extname icon
notify 'foo', :icon => :jpeg

# Image path icon
notify 'foo', :icon => 'images/error.png'

# Icon path icon
notify 'foo', :icon => 'images/error.icns'

# Extname from path icon
notify 'foo', :icon => 'foo.rb'

# App icon
notify 'foo', :icon => :Safari