escape_string before calling Graphviz

[lukem12345]

Currently, strings are not escaped before being passed to Graphviz.

Attempting to call to_graphviz on a graph with improperly-escaped strings may resulting in an stdin pipeline error.

Instead of constructors of data structures being responsible for escaping their String attributes, it would be best for string-escaping to happen just before being passed off to Graphviz output.