create a simple helper for basic planar use

[Fil]

as per https://github.com/d3/d3-drag/issues/27#issuecomment-342212672 ; a starting point might be https://bl.ocks.org/Fil/a034e32156b7b25ee9054c1bc50dc0ca without the fancy & random stuff

[mbostock]

Wow, nice example!