Disabling color through environment variables (#598)

* This patch adds support for disabling color through environment variables.
- NO_COLOR: https://no-color.org/
- CLICOLOR,CLICOLOR_FORCE: https://bixense.com/clicolors/ ( [logrus built-in](6699a89a23/text_formatter.go (L46)) )

* use viper/cobra and add documentation. (https://github.com/containrrr/watchtower/pull/598#pullrequestreview-463814669)

Co-authored-by: Simon Aronsson <simme@arcticbit.se>

internal/flags/flags.go

@@ -140,7 +140,12 @@ func RegisterSystemFlags(rootCmd *cobra.Command) {
 		"",
 		viper.GetString("WATCHTOWER_HTTP_API_TOKEN"),
 		"Sets an authentication token to HTTP API requests.")
-	
+	// https://no-color.org/
+	flags.BoolP(
+		"no-color",
+		"",
+		viper.IsSet("NO_COLOR"),
+		"Disable ANSI color escape codes in log output")
 	flags.StringP(
 		"scope",
 		"",