Add scripts for .travis.yml to append config options

buildroot/bin/opt_add

@@ -0,0 +1,3 @@
+#!/usr/bin/env bash
+
+eval "echo \"#define ${1} ${2}\" >>Marlin/Configuration.h"

buildroot/bin/opt_add_adv

@@ -0,0 +1,3 @@
+#!/usr/bin/env bash
+
+eval "echo \"#define ${1} ${2}\" >>Marlin/Configuration_adv.h"