default variable names or indices for forceNetwork() arguments

The arguments in forceNetwork() that specify which variables/columns in the data frames to use could have sensible defaults. At the moment, forceNetwork() will error if all of the arguments below are not set. This is particularly annoying when your data has no concept of "Group", but you are forced to put something in there just so the function works. Defaults could be indices (preferable?), i.e. 1st column of the Links data frame is assumed to be the 'Source' column by default, or could be strings, i.e. Source = 'source'.

These are the arguments to consider making defaults for: Source, Target, Value, NodeID, Group