Proposal: Share dot representation with graphviz-rust

[AlienKevin]

Thanks for developing this awesome crate! It'd great if the underlying dot representation can be shared with another great library called graphviz-rust. Sharing the representation can combine the intuitive interface of GraphWalk with the many attributes provided by grpahviz-rust. Currently, I'm using this library to display Finite Tree Automata as graphs. The output looks great but I also wanted to set the rankdir attribute for the generated Digraph to BT. Since I can't find a way to set rankdir using this library, I had to render the output to a Vec<u8> and parse the bytes from graphviz-rust. Then, I had to add the attribute there and finally write the result to a file. It would be great if this intermediate conversion is eliminated.