Add richer formatting to dot output

[sunshowers]

Use the DotWrite as the output, and allow arbitrary attributes to be set.

Probably going to require that the DotWrite become &mut DotWrite, so it's a breaking change.

[sunshowers]

cc @rexhoffman in case you want to tackle it!

There's probably a way this can be done without an API breakage -- lmk if you want to talk about it.