EjectMenu: use of modifier keys is undocumented and broken if menu not open

[EjectMenu](http://www.hammerspoon.org/Spoons/EjectMenu.html)'s handling of modifiers keys is undocumented and distracting if the menu isn't open:

1. If a menu item is selected while a modifier key is pressed, then instead of ejecting the volume, the volume is opened in the Finder (see [execMenuItem()](https://github.com/Hammerspoon/Spoons/blob/master/Source/EjectMenu.spoon/init.lua#L159). This behavior is undocumented as far as I can tell.
2. The menu icon responds to Command being pressed by changing its icon (see [changeEjectMenuIcon()](https://github.com/Hammerspoon/Spoons/blob/master/Source/EjectMenu.spoon/init.lua#L233)). However this happens even if the menu is not already open, which is distracting (especially since clicking on menu items with the Command key pressed does nothing).