Allow default event handling on excluded classes

[udevnl]

Default event handling is prevented for the on-mousedown event of excludedClasses.

For example, dragging with the mouse down and that this causes scrolling of a browser window is default behavior. This is being prevented.

This is bothersome when you also use other frameworks that rely on default behavior.

[Adraub]

Hi, Thanks for this lib! Would it be possible to merge this commit on a new version? It is quite useful when you try to use simple things such as input in html, who won't work if they are child of a draggable scroll. I use it to emulate single touch and I already applied the commit on my project. Having drag-scroll-excluded-classes on the doc would also be as mikkoviitala suggests, i did'nt knew it was possible :)