Add suport for "standard" NO_COLOR env variable (#508)

See https://no-color.org

nnn.1

@@ -410,6 +410,8 @@ separated by \fI;\fR:
 .Pp
 \fBnnn:\fR this is a special variable set to the hovered entry before executing
 a command from the command prompt or spawning a shell.
+.Pp
+\fBNO_COLOR:\fR if defined, disable ANSI color output.
 .Sh KNOWN ISSUES
 .Nm
 may not handle keypresses correctly when used with tmux (see issue #104 for

src/nnn.c

@@ -1524,7 +1524,7 @@ static bool initcurses(void *oldmask)
 #endif
 	curs_set(FALSE); /* Hide cursor */
 
-	if (!(g_states & STATE_NOCOLOR)) {
+	if (!(g_states & STATE_NOCOLOR || getenv("NO_COLOR"))) {
 		start_color();
 		use_default_colors();