feature: Allow extra properties per tracking handler

[pedrocarmona]

Hi @ankane,

Yet another way to allow custom properties on events.

Usage:

import "ahoy"

function otherEventProperties(element, event) {
  return {
    test: "here"
  };
}

ahoy.trackClicks("a, button", otherEventProperties);

Notice:
Internal functions of ahoy are not exposed, so not possible to use getClosest(), page(), presence(), etc.

[pedrocarmona]

Could also remove visitParams and have a default hook for $view...

[bborn]

👍 on this. @ankane any thoughts on merging this in?