filtering feature added

inner objects of draggables are now excludable/filterable to prevent the drag-start when tapping on these inner elements

options.filter can be an array ob selector strings or a comma-separated string of selectors

usecase: buttons within these draggables have another functionality like 'delete this draggable'

NOTE: i couldn'd find another way to do this with the existing API. so i figured you might wanna extend it by this feature