Allow users to configure how edges are displayed

[jameslamb]

Currently in R/AbstractGraphReporter.R, we hardcode the color to use when plotting edges:

plotDTedges[, color := '#848484']

To complete this issue, change pkgnet to allow people to configure this. The ideal solution should NOT add new arguments to any of the main workflows documented in our vignette (e.g. we don't want to see CreatePackageReport('lightgbm', edge_color = 'red')) but should instead add some option like $set_edge_visualization_options() (or probably something shorter).