Support for Graph Modeling Language

This PR adds support for the Graph Modeling Language as requested at #1559.
There are many [users of GML on GitHub](https://github.com/search?p=5&q=extension%3Agml+graph&ref=searchresults&type=Code&utf8=%E2%9C%93).

It uses the `.gml` file extension which conflicts with Game Maker Language.
The Bayesian classifier seems to do a pretty good job at distinguishing the two.
I tested my PR on:
- [branjbar/compat_modeling](https://github.com/branjbar/compat_modeling)
- [gephi/gephi-toolkit-demos](https://github.com/gephi/gephi-toolkit-demos)
- [h3rb/gml-pro](https://github.com/h3rb/gml-pro)
- [graphstream/gs-algo](https://github.com/graphstream/gs-algo)
- [sujitpal/mlia-examples](https://github.com/sujitpal/mlia-examples)