A Simple Parser for Graphviz DOT language

Parse DOT into GraphAST, which can then convert to a graph in the petgraph library.

Still work-in-progress, pull requests are welcomed!

A few TODOs

• [ ] Handle comments/spaces correctly
• [ ] Support subgraphs and attribute statements
• [ ] Convert node/edge labels into petgraph
• [ ] Support edges with multiple RHS
• [ ] Support node_id with ports