Prevent double initialization

[rodneyrehm]

This will cause the menu to go haywire:

$.contextMenu({
  selector: '.selector',
  // ...
});
$.contextMenu({
  selector: '.selector', // this selector is already registered
  // ...
});

this should be prevented (or silently corrected). Two options

a) do a $.contextMenu("destroy", ".selector"); internally (seemless overwrite)
b) throw an error