Only accept `'on' + action + phase` for listeners

when adding them with the object passed to Interactable#<actionable>

interact(target).draggable({
  onstart: function () {},      // will work as always
  start: function () {}         // will no longer work
})

interact(target).dropzone({
  ondrop: function () {},       // will work
  drop: function () {},         // won't work

  ondragleave: function () {},  // will work
  dragleave: function () {}     // won't work
})