Function format_pct 

pub fn format_pct(pct: f64) -> String

Format a fractional change as a percentage string.

assert_eq!(perfgate_render::format_pct(0.1), "+10.00%");
assert_eq!(perfgate_render::format_pct(-0.05), "-5.00%");
assert_eq!(perfgate_render::format_pct(0.0), "0.00%");