Hide placeholder text when the input is invisible

In case the input is inside an inactive tab for instance.

It would also be great if you could provide an API to force refreshing the placeholder positioning/visibility. window resize is not the only event that could change the positioning of an input and currently I'm forced to do something like `$(window).trigger('resize.placeholderShim')` to force `placeholderShimRefresh` to be called.

It would be great if I could do something like `$('[placeholder]').placeholderShim('refresh')`.