Multiple output policies

[sharkdp]

• Allow --output to appear exactly once or exactly N times, enabling several new use cases like hyperfine --output=cmd1.log cmd1 --output=cmd2.log cmd2 or hyperfine --output=null cmd --output=./file cmd.
• In the --help text for --output, mention the HYPERFINE_ITERATION environment variable for users who want to log each and every iteration.

closes #529

[StripedMonkey]

Thank you for your hard work. It's much appreciated!

[sharkdp]

Thanks for patiently waiting two years for this 😄