feature: render output using DOT

[SethTisue]

trafficstars

some WIP on this is at #51 if someone wants to finish it up.

@jrudolph concluded "doesn't seem particularly useful on reasonably sized code bases, takes dot ages to generate a graph and the result is hardly usable"

but if we had source-file-level and/or package-level aggregation (#56), the graphs for that would probably be usable