Add option to format output as CSV

[amichair]

It would be really great if wrk had a command line option to output all of its output information (including the optional latency info, if specified) in a single long CSV line rather than the current human-readable format. This would make it much easier to use the results in further analysis, graphs, multi-configuration comparisons, etc.

A more complex option would be to provide a custom format string with placeholders for all the different pieces of data in the output so anyone can adjust it to their precise needs, however a stock CSV line output with all data should be simple enough to further process in any scripting or programming language (much less effort than parsing the current human-readable output).

[wg]

This is possible with a custom Lua script with a done(summary, latency, requests) function, see the report.lua example for a starting point: https://github.com/wg/wrk/blob/master/scripts/report.lua