Increase functions: 1. export JSON network; 2. remove nodes from network

[mildpiggy]

[cannin]

@mildpiggy Thanks for the contribution; sorry for the delayed response. Would you be willing to add to the basicDemo as a way to test out the additions?

https://github.com/cytoscape/cyjShiny/blob/master/inst/demos/basicDemo/app.R

Is removeSelection the same as clearSelection in the demo? Also, I am not sure how the filename gets used for the JSON file download.