Allow selection of plots / suppression of iteration times

[dhardy]

I've been using a large sample size (100k to get a time distribution with randomised parameters), and the generated plots get large (plots including all iteration times for one benchmark are around 10 MiB, the sum of outputs for one benchmark group is 1.4 GiB). I don't need all of the plots, they take significant time to generate, and attempting to load index.html for a group and scroll down the page crashes Firefox. But I do want some of the plots (at least the main violin.svg comparing the PDF of each method).

Iteration times aren't really useful with this many samples and could be disabled automatically. This, and everything including them (pdf.svg), account for almost all the output size, so simply having an on/off switch for iteration times should be enough.

I guess the correct place for this configuration would be cargo-criterion command-line parameters.