Support waiting between trials and also ignoring some initial trials

This adds support for two things which are helpful in practice:

- Ignoring the first `n` out of `m` total trials in order to avoid JVM startup behavior.
- Adding a pause between trials, which allows for things like buffer cache write-back.

There is also a bug fix - in an earlier commit I added something to report the time of the first and last trial, but those were assessed only _after_ the list had been sorted. So "first" was always the fastest and "last" was always the slowest! Whoops :)