Fullscreen overlay mode does not exit on ESC key

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:
```javascript
// 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?

Start in fullscreen.js at line 92 and compare its keyboard handling with Flickity's keyboardHandlers in core.js at line 787. Reproduce the fullscreen overlay behavior, then verify that pressing Escape exits the overlay and that the relevant keyboard handler uses the expected event value.

Written by the indexing model from the issue text.