Allow specifying custom graphviz arguments

[alek3y]

This should fix #88.

It reads the arguments option from the config file (or defaults to ["-Tsvg"]) and passes it down to call_graphviz.
For example:

[preprocessor.graphviz]
arguments = ["-Tsvg", "-Grankdir=LR"]

Note: I haven't had much experience with lifetimes, so I trusted the compiler more or less.