[Enhancement] Consider an alternate modifier for closing the tree

[joshgold22]

On Mac OSX it is difficult to generate a middle-click without installing extra software (like BetterTouchTool or specialized apps with names like MiddleClick)
Could you add an option to support Mouse Button 2 (control-click) instead if desired?
I would expect the context menu to still come up when clicking anywhere else but the Close button.

Option-click is another possibility to consider allowing as well.
(I would avoid Shift or Command, because of their use in selecting/deselecting tabs.)

This request came from a discussion here:
piroor/treestyletab#3230