Add export options

[sharkdp]

It would be great if we could export benchmark results in different formats:

• Markdown for easy integration into README files
• JSON for further processing
• CSV for simple plotting (gnuplot) -- header + one line per benchmark with columns mean, stddev, min, max

Each of those could be new command line options (hyperfine --export-csv my-benchmark.csv).

[stevepentland]

@sharkdp I'll take this one on. Do we want to split this into multiple sub-tasks for output type? That will allow for incremental progress. And, when specified, will we retain all standard terminal output also?

[sharkdp]

Do we want to split this into multiple sub-tasks for output type?

Sounds good. We can still keep this one for now to discuss the general plan.

And, when specified, will we retain all standard terminal output also?

Yes, I think so. In addition, we would collect all results in some data structure and then write everything to the specified file at the end of the benchmark.

[sharkdp]

Closed via #41. Markdown and JSON export can be tracked separately.