Register event handlers for an event name to ref.

@param {Element} el - DOM element.
@param {string} eventName
@param {array|function} eventHandlers - Handler function or array of handler functions.
function addEventListeners(el, eventName, handlers) {
  var handler;
  var i;

  if (!handlers) {
    return;
  }

  // Convert to array.
  if (handlers.constructor === Function) {
    handlers = [handlers];
  }

  // Register.
  for (i = 0; i < handlers.length; i++) {
    el.addEventListener(eventName, handlers[i]);
  }
}