Enhancements

(includes removal of dependency not in use, the exact same commit as in my other PR ( #7 )
- Add possibility to specify starting position
- Add possibility to add extra functionality to onMouseDown event. Before this PR, it is blocked by the library, and it is quite useful to be able to attach extra functionality on that event. For example, in an application with multiple draggable modals, you'd want to change each modals z-index based on the mousedown event on the first child.