Tooltip/popover: Allow viewport option to be a function

Created by: Intrepidd

It might be really useful to be able to pass a function to the viewport option in some cases, just like that :

$('body').tooltip({
  selector: '[data-toggle="tooltip"]',
  viewport: function($element) {
    var viewport = $element.closest('.tooltip-viewport');
    return (viewport.length > 0 ? viewport : $('body'));
  }
});

I implemented that in this PR.

I'm open to suggestions, I'm not sure if it's a good idea to accept either a string, an object, or a function. (Did not update the doc yet because of that)

Regarding tests, I noticed that there are no tests asserting that using functions instead of string work (placement for instance), so I copied an existing viewport test, you tell me if that's enough.

Thanks