Add measurement of vividness

[Bisaloo]

As defined in https://doi.org/10.1242/jeb.204487.

The first author's python code is licensed as CC-BY-NC-SA, which means we can't directly translate into R for pavo (because of the NC clause, which is absent from the GPL): https://github.com/lucas-wilkins/lemonsauce

• Add plot.vismodel()