login: extend configuration instead of overwriting it

lib/login.js

@@ -77,6 +77,12 @@ async function register () {
 }
 
 export default async function () {
+  const path = resolve(os.homedir(), '.now.json');
   const loginData = await register();
-  return fs.writeFile(pathJoin(homedir(), '.now.json'), JSON.stringify(loginData));
+  let existing = null;
+  try {
+    existing = await fs.readFile(path, 'utf8');
+  } catch(err){}
+  const config = Object.extend({}, existing, loginData);
+  return fs.writeFile(path, JSON.stringify(config));
 }