Add adapters for common graph formats

[Matt Cloyd]

"Not just for Neo4j anymore!"

Cypher was a major impetus for building Aspen...but it's not the only graph visualizer out there.

Getting the Aspen Editor off the ground, however, is taking a while. For the first public demos, I'd rather have everything be client-side rendered and not have to get posted to a server, then a somehow-multitenanted free Neo4j database, then returned, potentially with Cypher errors.

It would be much simpler to start if the Aspen Editor output graph data in the JSON-encoded graph format used by Sigma.js & G6, and/or GEXF. That way, we could just pipe the JSON output to Sigma or G6 instead of having to do all the Neo4j setup. Later, we can add Neo4j support.

Relatedly, #19 is asking for a feature that might be well-served by adding an ActiveGraph adapter (perhaps one that builds the code but lets the Rails app run it).