tlog: respect NO_COLOR

Fixes https://github.com/rfjakob/gocryptfs/issues/617

Documentation/MANPAGE.md

@@ -673,6 +673,13 @@ the `-nofail` option for details).
 
     /tmp/cipher /tmp/plain fuse./usr/local/bin/gocryptfs nofail,allow_other,passfile=/tmp/password 0 0
 
+ENVIRONMENT VARIABLES
+=====================
+
+### NO_COLOR
+
+If `NO_COLOR` is set (regardless of value), colored output is disabled (see https://no-color.org/).
+
 EXIT CODES
 ==========
 

internal/tlog/log.go

@@ -106,7 +106,9 @@ var Warn *toggledLogger
 var Fatal *toggledLogger
 
 func init() {
-	if term.IsTerminal(int(os.Stdout.Fd())) {
+	// Enable color output if we are connected to a terminal and NO_COLOR is
+	// unset ( https://no-color.org/ ).
+	if _, nocolor := os.LookupEnv("NO_COLOR"); !nocolor && term.IsTerminal(int(os.Stdout.Fd())) {
 		ColorReset = "\033[0m"
 		ColorGrey = "\033[2m"
 		ColorRed = "\033[31m"