Allow menu to be shown on 'contextmenu'

[thomasstjerne]

If the click event is used for other purposes (i.e. set in the node-styler option), then it would be convenient to be able to show the menu on the "contextmenu" event rather than the "click" event.

Instead of the show-menu option being a boolean, it could be either "click", "contextmenu" or null (defaulting to "click").

Thanks for all your work!