visualize(::AtomGraph) should write to file

[jonathanmfung]

Port of Chemellia/ChemistryFeaturization.jl#109

This should be easy enough. I think @sakshi-satyanand is going to take a crack at this :)

[jonathanmfung]

Since visualize(::AtomGraph) uses GraphPlot.gplot(), a user would have to use the Compose.jl library to save the figure.

It would be possible to reexport Compose.draw(), but that would add another dependency. Additionally, GraphPlot.jl does not reexport themselves. So, I would err on the side of leaving it up to the user.

The implicit return defined in #3 should be able to work with Compose.draw(), though I'm a bit rusty with Julia's handling of returns and plots.