Add cleanup command which is executed at the end of each benchmark

[colinwahl]

Fixes #91.

I am still a beginner at Rust, so I'm very open to feedback. Thanks!

[sharkdp]

Thank you very much for your contribution!

I will do a full review soon, but what I've seen so far looks great. I think we should work a little bit on the --help text to clearly explain what the difference from --prepare is. The prepare-command will be executed before each benchmarking run (several times for every benchmarked command) while the cleanup command will only be called once after a set of benchmark runs (once for every benchmarked command).

[sharkdp]

Thank you very much. The rest of the changes looks great!

[sharkdp]

As stated previously, we should clarify the help text here.

[colinwahl]

I agree it needs some clarification, how does this sound?

"Execute CMD after the benchmarking runs for each individual command to be benchmarked. This is useful if the commands to be benchmarked produce artifacts that need to be cleaned up."

[sharkdp]

Sounds great!

[colinwahl]

Ok! I have updated the code.

[sharkdp]

Thank you!