Add a method converting network to data frames

[mbojan]

Move a method from https://github.com/mbojan/intergraph

[mbojan]

Closing because 'intergraph' will take advantage of network::as.data.frame() method.

[knapply]

Moved to mbojan/intergraph#30 ...

[mbojan]

Thanks @knapply , indeed! Would you care moving your comment to a new issue at https://github.com/mbojan/intergraph? The package need some review but I'd love some input/PR on this once I get to that

[knapply]

Done and done!