Use local var `environment` rather than `NODE_ENV`

Traditionally, NODE_ENV refers to an environment variable. For clarity,
let's keep it that way and don't reuse it in the renderer. Also, add a
note about explicitly overriding env vars for node-config.

// FREEBIE

js/background.js

@@ -9,7 +9,7 @@
     };
 
     console.log('background page reloaded');
-    console.log('NODE_ENV', window.config.NODE_ENV);
+    console.log('environment:', window.config.environment);
     extension.notification.init();
 
     var initialLoadComplete = false;

main.js

@@ -27,7 +27,10 @@ if (shouldQuit) {
 }
 
 const package_json = JSON.parse(fs.readFileSync(path.join(__dirname, 'package.json'), 'utf-8'))
-process.env.NODE_ENV = package_json.environment || process.env.NODE_ENV || 'development';
+const environment = package_json.environment || process.env.NODE_ENV || 'development';
+
+// Set environment vars to configure node-config before requiring it
+process.env.NODE_ENV = environment;
 process.env.NODE_CONFIG_DIR = path.join(__dirname, 'config');
 const config = require('config');
 
@@ -70,7 +73,7 @@ function createWindow () {
       version: package_json.version,
       buildExpiration: config.get('buildExpiration'),
       serverUrl: config.get('serverUrl'),
-      NODE_ENV: process.env.NODE_ENV
+      environment: environment
     }
   }))