Adds `refresh` method to re-fetch `[data-modal]`

[zackkatz]

See #6 for more information.

I know this isn't the best solution, but I'm not a great JS developer.

Another alternative would be to add something like this:

/**
 * @param {Element} item
 */
ARIAmodal.addModal = function ( item ) {
	modal.push( item );	
};

and then refactor setupModal so it runs only on one modal at a time, then add a new setupModals method that would loop through all the items in the modal array and run setupModal on them.

Again, I'm not great at JS. Thank you for the script, and please let me know if I can help with the PR.

Closes #6