Output .dot format for graphviz

[cscherrer]

Like this:

[devmotion]

Reminds me of https://github.com/TuringLang/IRTracker.jl/blob/master/src/graphviz.jl.

[cscherrer]

Ooh nice, and it can do clusters! Maybe this spin out to a general-purpose library?

[cscherrer]

cc @phipsgabler

[phipsgabler]

Oh, that's really 90% copied from Catlab and adapted to my requirements back then. It was the only interface I had found at this level of granularity. The better idea would be to ask the Catlab people for spinning out stuff, since they do a much better job of maintainance.