[FEAT] (right-click contextmenu) Added support for contextmenu event.
[FEAT] (listen events) Ability to listen to events, specified through extension points. The syntax requires a double array, one for each call to mark_event, and another for multiple parameters.
  Example: "bar_event": [
      ['contextmenu', function(s) { alert('context ' + s.vars.category.value); }],
      ['click',       function() { alert('click'); }]
  ]