Feature request: allow limiting very long-running tests

I would like a flag that lets me say "never spend more than X seconds on a benchmark; if the time limit is exceeded compile statistics with runs you've done".

I assumed from help that this was what `-L` did, but I was wrong (I think those docs could be improved too; `-L` seems to run a benchmark for some fixed amount of time which we wouldn't normally call a "time limit"... I wonder if it was originally meant to do what I thought it did...).