[Feature request] Display copy speed in progressbar

[desbma]

Thanks for making this project.

I read the LWN article on copy, and was wondering if someone had the courage to rewrite cp like for example what the author of ripgrep did for grep.

I'll open a few issues with my first thoughts using xcp, feel free to close/ignore them if you disagree.

It would be great to display the copy speed (in KB/s or MB/s) at the end of the progress bar.

I know that you are using the indicatif crate, and this is not built in, but that would be a valuable addition nevertheless.