Add "Repeat" and "GroupRepeated" options

[WillyPillow]

This adds a Repeat option that automatically duplicates testsets. This is useful for ABX tests, where multiple attempts are necessary to get statistically significant results.

However, RandomizeFileOrder shuffles the identical tracks apart, which may or may not be desired, so another option GroupRepeated is added to mitigate that.

I regenerated README.html using pandoc as per the commands in tools/generate_docs.sh, hope I'm not doing it wrong.