WIP Support for HTML labels

There currently is no support for HTML table labels http://graphviz.org/content/node-shapes#html 

I was curious for this support so wrote these small failing tests.

- [ ] Fix failing test for normal title (trivial to fix)
- [ ] Fix failing test for record title (is all escaping properly esp. ports: http://graphviz.org/content/node-shapes#record)
- [ ] Fix failing test for HTML title (this is tricky due to the current escaper)

Note I come from https://github.com/clue/graph-uml and https://github.com/clue/graph of which the latter has a graphviz rendering. The latter is about to split off graphviz into it's own project so maybe we can team-up ... not sure.

Feel free to close or fix for the tests :-)