Areas to work on:

* Utilise the generic graph class once it is finalised.

* Allow user to choose whether or not the graph is meant to be
  directed or undirected.

* Improve parsing to fully (or at least follow more closely) support Dot.

* Improve clustering/subgraph support.

* Use a PrettyPrinter rather than Show to generate Dot output.

* Improve Output support.

* Find and fix the handle closing bug with graphvizWithHandle.