Add $hintSelector variable

[ryanve]

This would allow customizing like

$hintSelector: '.hint';
$hintPrefix: 'hint--';

to use like

<div class="hint hint--top">example</div>

and make it easy to customize via .hint

[chinchang]

@ryanve Thanks for the pull request. I would just like to know if there was any use case of yours to change the selector of the tooltips?

[ryanve]

Of course :) I'm using aria-label for the tooltip content so the only way to customize all tooltips would be by the somewhat cumbersome attribute selectors. Being able to use a simple base class like .hint seems cleaner and less error prone.