Add support of various graph output formats

[purohitsumit]

Current supported format is JSON. Add following 2 formats to increase re-usability in various graph mining tools, and add support of some out-of-box visualization such as Gephi.

What 2 formats were you referring to?

[purohitsumit]

1. Tab separated RDF file format
2. GEXF File format supported by Gephi Visualization.
   https://gephi.org/gexf/format/