Add new --sort option

[sharkdp]

This adds a new --sort option to choose the way in which the results in the speed comparison and the markup exports are ordered.

Default behavior, --sort auto or --sort mean-time:

With --sort command

Markdown export with --sort mean-time:

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
sleep 0.1	101.1 ± 0.5	100.6	103.1	1.00
sleep 0.2	200.9 ± 0.3	200.7	201.9	1.99 ± 0.01
sleep 0.3	301.0 ± 0.2	300.8	301.3	2.98 ± 0.01

closes #614 closes #601

[SUPERCILEX]

This is awesome, thanks!