make vex-close a button to help with accessibility

[WesKetch]

Related to #133

Hello,

I have been working on making the vex modal more accessible. By using a button to close the modal instead of a div, someone can now focus on the button using the tab key, and close the button using the enter key. I have gone through all the themes to make sure the styles are consistent. CSS is not my strongest skill so I may be a little off on some of them.

Thanks!

[adamschwartz]

👍

[modean]

You could achieve the same with something similar to the below code snippet. The point is to make the screenreader think it deals with a button. Tho, from a markup point of view I like it more to have more expressive markup so that you immediately know the meaning of a markup element if you see it for the first time. That is if you want to provide a button use <button />.

    $('.vex-close').attr('role', 'button');
    $('.vex-close').attr('tabindex', '0');
    $('.vex-close').attr('aria-label', 'close');

Also use the aria-label role instead of title. See:

http://wesbos.com/times-html-entity-close-button/

Any plans to merge this PR with #133 to get both into the master branch?