Return this from customize for chaining.

lib/index.js

@@ -57,7 +57,7 @@ function Config(parent, defaultEnvVar, initialCustomizer) {
   };
 
   config.customize = function (newCustomizer) {
-    newCustomizer(config);
+    newCustomizer(this);
     if (!customizer) {
       customizer = newCustomizer;
     } else {
@@ -72,6 +72,8 @@ function Config(parent, defaultEnvVar, initialCustomizer) {
     Object.keys(environments).forEach(function (envName) {
       environments[envName].customize(newCustomizer);
     });
+
+    return this;
   };
 
   config.set = function (setting, value) {