add branch config

Author: shmargum

.gitignore

@@ -1 +1,2 @@
 node_modules/
+*.swp

lib/build.js

@@ -13,18 +13,26 @@ function getBabelTarget(envConfig) {
   return unknown ? "6.10" : current.replace(/^nodejs/, "");
 }
 
-function webpackConfig(dir, additionalConfig) {
-  var config = conf.load();
+function loadEnvironment(config) {
   var envConfig = config.build.environment || config.build.Environment || {};
-  var context = (
+  var contextConfig = (
     process.env.CONTEXT &&
     config.context &&
     config.context[process.env.CONTEXT] &&
     (config.context[process.env.CONTEXT].environment || config.context[process.env.CONTEXT].Environment)
   ) || {};
-  Object.keys(context).forEach(key => {
-    envConfig[key] = context[key];
-  });
+  var branchConfig = (
+    process.env.BRANCH &&
+    config.context &&
+    config.context[process.env.BRANCH] &&
+    (config.context[process.env.BRANCH].environment || config.context[process.env.BRANCH].Environment)
+  ) || {};
+  return merge(envConfig, contextConfig, branchConfig);
+}
+
+function webpackConfig(dir, additionalConfig) {
+  var config = conf.load();
+  var envConfig = loadEnvironment(config);
   var babelOpts = { cacheDirectory: true };
   if (!fs.existsSync(path.join(process.cwd(), ".babelrc"))) {
     babelOpts.presets = [