A drag that was 'caught' by an iFrame can be stopped by clicking the swatch

Assume the situation where minicolors is initiated on an input field in the parent frame, and a iframe is positioned somewhere. Now we open the swatch and want to drag to another color. If you now drag out of the swatch; drag over the iframe and release the mouse button: the drag event is still going on (this is known behaviour). The swatch now follows the mouse even though we are no longer holding any button. 

Now I expect that a click on the swatch will update the position and cancel the drag. This is however not the way minicolors works. This PR fixes this behaviour.

To conclude.. thanks for this awesome library :)!