Add labels to graphs generated by `plotRuns()`

[gothub]

The DiagrammeR package is used to make lineage trace graphs, and I haven't found a way to make a legend, so instead the arrows connecting executions and files should be annotated.

The default would use the inverse relations of ProvONE, which are prov:wasUsedBy and prov:generated, in which case the arrows point in the direction of data flow, i.e. from older objects to newer, which I believe will be more easily understood to users of this package.

The user would be able to override this default and specify that the regular ProvONE labels are used, such as prov:used and prov:wasGeneratedBy, in which case the arrows would point backward, from descendants to antecedents.