fixes #258 chainable .required() (#259)

* fixes #258 chainable .required()

* fixes #258 use var instead of let for travis builds

README.md

@@ -180,6 +180,24 @@ Declares a set of string keys to be mandatory, and throw an error if any are mis
   nconf.required(['keya', 'keyb']);
   // Error: Missing required keys: keyb
 ```
+You can also chain `.required()` calls when needed. for example when a configuration depends on another configuration store
+
+```js
+config
+  .argv()
+  .env()
+  .required([ 'STAGE']) //here you should have STAGE otherwise throw an error
+  .file( 'stage', path.resolve( 'configs', 'stages', config.get( 'STAGE' ) + '.json' ) )
+  .required([ 'OAUTH:redirectURL']) // here you should have OAUTH:redirectURL, otherwise throw an error
+  .file( 'oauth', path.resolve( 'configs', 'oauth', config.get( 'OAUTH:MODE' ) + '.json' ) )
+  .file( 'app', path.resolve( 'configs', 'app.json' ) )
+  .required([ 'LOGS_MODE']) // here you should haveLOGS_MODE, otherwise throw an error
+  .add( 'logs', {
+    type: 'literal',
+    store: require( path.resolve( 'configs', 'logs', config.get( 'LOGS_MODE' ) + '.js') )
+  } )
+  .defaults( defaults );
+```
 
 ## Storage Engines
 

lib/nconf/provider.js

@@ -363,7 +363,7 @@ Provider.prototype.required = function (keys) {
   if (missing.length) {
     throw new Error('Missing required keys: ' + missing.join(', '));
   } else {
-    return true;
+    return this;
   }
 
 };

test/nconf-test.js

@@ -48,9 +48,10 @@ vows.describe('nconf').addBatch({
         nconf.set('foo:bar:bazz', 'buzz');
         assert.throws(nconf.required.bind(nconf, ['missing', 'foo:bar:bazz']), Error);
       },
-      "should return true if all required keys exist": function() {
+      "should return the provider if all required keys exist": function() {
+        var Provider = nconf.Provider;
         nconf.set('foo:bar:bazz', 'buzz');
-        assert.isTrue(nconf.required(['foo:bar:bazz']));
+        assert.isTrue(nconf.required(['foo:bar:bazz']) instanceof Provider);
       }
     }
   }