Add `jitter` option to avoid overlapping node labels

[zachary-foster]

Ideally, this would actively avoid overlaps using some kind of annealing algorithm. Perhaps modeling the distance between the label center and the node center as a spring balanced against repulsion from other labels. Maybe look at how ggplot2 does it.

This came up in the Google groups forum:

https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/metacoder-discussions/CqtzrnUFOi4/Q_9DVPUgBQAJ

[zachary-foster]

Done. option repel_labels