Filter functions

Katavorio can now understand drag filters that are functions. Filter functions receive two parameters: the event that caused the function to trigger, and the draggable on which the event was triggered. They should return either true (to stop the default drag behavior) or false (to continue it).
filterExclude works with filter functions in the same way that it works with filter strings: it flips the true/false value of the filter.
No special consideration of handles has been implemented.
I have included basic get/set tests and added an element to the demo page (though it's poorly positioned).