feat(draggable): added dragWithin option to constrain drags

Draggable option `dragWithin` enables the browser to limit the area
in which a draggable element can be positioned.

This initial implementation is extremely simple and is likely to be
improved upon in time.
docs(demo): added demo of drag-within constraints usage
Merge pull request #13 from caitp/issue-13

feat(draggable): added dragWithin option to constrain drags