Provide graphml export option

In my mind one of the key things (besides visualization) Sourcetrail provides is a useful graph structure for source code that can be used for various types of analysis, without having to go through the pain of writing a clang analyzer yourself. However, since sourcetrail doesn't have plugins (#1076), writing your own analysis tools is a bit of work, and may not let you use other kinds of graph analysis tooling.

I've started on making a graphml exporter at https://gist.github.com/virtuald/95ecfa5047671f008395f5781a359cc7 , but it seems that getGraphForNodeTypes and getGraphForAll don't actually seem to create a full in memory graph.