Add support for Sigma graph visualization library

[nathancarter]

Add support for sigmajs in addition to cytoscape for graph drawing.

It's "a JavaScript library dedicated to graph drawing. It makes easy to publish networks on Web pages, and allows developers to integrate network exploration in rich Web applications."

This requires also extending ConvertGraphForTool to support the new sigma format.

(I'm adding this issue as part of an effort to make my to-do list for this project public.)