Add option to clear all old plots

[tbekolay]

It's pretty a pretty common annoyance to generate plots and then not know which ones are new and which ones you generated some other time. Also, it's common when iterating on a test you're writing to change the parametrizations and therefore leave a bunch of stale plots behind that are no longer valid. All to say that it would be nice to have a command line option that would remove all of the existing plots in the plot directory before generating the plots for the current run. Something like

pytest --plots-clear --plots

would clear all the plots first, then generate new ones.