v1.9.0

Updates to network_p2p(), with breaking changes to the argument, addressing issue #225:

• network_p2p() now has a centrality argument for computing and visualizing centrality as node sizes in the network plot
• the algorithm argument is renamed to layout for better intuitiveness
• Improved consistency and intuitiveness of the API, with style argument now controlling the network plotting mechanism and return argument controlling whether plots are generated interactively or saved as PDF
• Added a large number of community detection algorithms from igraph

network_summary() adds the ability to return pagerank.

This version also includes some minor dependency maintenance done to create_dt().