Fullscreen overlay mode does not exit on ESC key

[indigoxela]

First of all: many thanks for this cool library, Flickity really rocks. 🌟

While playing with the fullscreen plugin I encountered a problem with keyboard handling, though.
The Escape key doesn't close the overlay.

Here's the code: https://github.com/metafizzy/flickity-fullscreen/blob/master/fullscreen.js#L92

However, Flickity.keyboardHandlers doesn't expect the number from event.which, but apparently the string from event.key: https://github.com/metafizzy/flickity/blob/master/js/core.js#L787

Maybe that behavior changed at some point in the past...

Following fixes it for me:

// ESC key closes full screen
Flickity.keyboardHandlers['Escape'] = function() {
  this.exitFullscreen();
};

Is there interest in a pull request, or is this trivial change not worth it, anyway?