Benchmark time divisions

**Description**
Working on an audio application, one of the things i'd like to measure is the difference in performance depending on the size of the audio buffer. That is, i want to benchmark the processing time per audio sample when i call my `process` function with 1, 256, or 1024 samples at a time.
As the function i benchmark has to be the main `process` function, i have no way to get the per sample time, except for manually dividing the reported result, which isn't very handy when experimenting with a lot of small changes.

So, could we get a `divisor` or similarly named option, maybe in the `Chronometer` class, so it can be used with the `BENCHMARK_ADVANCED` macro.