Add an "output format" option and parser

[ChrisCummins]

It would be nice if srtime supported custom output formatting, similar to something like the date (1) util.

Example usage: srtime -f "mean: %m, variance: %v, iterations: %i" ./a.out Expected output:

mean: 1.100s, variance: 0.23, iterations: 20

[ChrisCummins]

Here's my work in progress ideas for supported format sequences:

%T total runtime
%i num of iterations
%m mean runtime
%l lower confidence interval
%u upper confidence interval
%v variance