fix(output): cache bg and fg colors by default

It's uncommon for users to change the colorscheme of their terminal from
a dark to light, or vice versa, while a program is running. This makes
Termenv query the terminal for colors too many times.

Enable color caching by default and give the program the option to
opt-out using WithColorCache(false).

output.go

@@ -69,6 +69,7 @@ func NewOutput(w io.Writer, opts ...OutputOption) *Output {
 		w:       w,
 		environ: &osEnviron{},
 		Profile: -1,
+		cache:   true,
 		fgSync:  &sync.Once{},
 		fgColor: NoColor{},
 		bgSync:  &sync.Once{},