Work with igraph package objects

[barryrowlingson]

It would be nice if this package could produce charts from igraph package objects. I might start writing some conversion functions for contribution if you think its worth it...

[christophergandrud]

I think that's a great idea!

Maybe the best structure would be to create a function that just takes igraph objects and turns them into data frames that d3ForceNetwork can work with comfortably. Kind of like what the d3Network JSONtoDF function currently does for JSON data.