Add option for reporting median runtime

[sleeepyjack]

I stumbled across this Slack thread recently when I was trying to measure small kernels with nvbench and got fluctuating results. As @senior-zero notes in the thread, the variance of the minimum runtime is usually smaller than that of the average runtime.

So it would be very useful if nvbench would provide a way to generate these values directly. We could also report additional metrics, such as max or median runtime.

[alliepiper]

min/max were added in #204. They are hidden by default, but there's an example in that PR that shows how to display them in the markdown output.

We don't compute the median, is there a strong usecase for this? It'd be somewhat expensive to compute as we don't currently sort the sample timings and I'd prefer to avoid that if possible.