added rateLimiter.py, by @felixhandte
added rateLimiter.py, by @felixhandte

this rate limiter avoid the problem of `pv`
which "catch up" after a blocked period
instead of preserving a constant speed cap.

Cyan4973/zstd,Cyan4973/zstd,Cyan4973/zstd,Cyan4973/zstd,Cyan4973/zstd