Try to detect color support, make color configurable

This commit tries to detect if users can render colors in their terminal
and only enables color logs if that is true.

It also adds no-color.org's NO_COLOR env var support to allow it to be
disabled.

cmd/headscale/headscale.go

@@ -4,6 +4,7 @@ import (
 	"os"
 	"time"
 
+	"github.com/efekarakus/termcolor"
 	"github.com/juanfont/headscale/cmd/headscale/cli"
 	"github.com/rs/zerolog"
 	"github.com/rs/zerolog/log"
@@ -11,10 +12,32 @@ import (
 )
 
 func main() {
+	var colors bool
+	switch l := termcolor.SupportLevel(os.Stderr); l {
+	case termcolor.Level16M:
+		colors = true
+	case termcolor.Level256:
+		colors = true
+	case termcolor.LevelBasic:
+		colors = true
+	default:
+		// no color, return text as is.
+		log.Trace().Msg("Colors are not supported, disabling")
+		colors = false
+	}
+
+	// Adhere to no-color.org manifesto of allowing users to
+	// turn off color in cli/services
+	if _, noColorIsSet := os.LookupEnv("NO_COLOR"); noColorIsSet {
+		log.Trace().Msg("NO_COLOR is set, disabling colors")
+		colors = false
+	}
+
 	zerolog.TimeFieldFormat = zerolog.TimeFormatUnix
 	log.Logger = log.Output(zerolog.ConsoleWriter{
 		Out:        os.Stdout,
 		TimeFormat: time.RFC3339,
+		NoColor:    !colors,
 	})
 
 	err := cli.LoadConfig("")