feat: Support nested locale keys.

Example:

```js
{
  model: {
    user: {
      name: 'Real Name',
      createdAt: 'Joined At'
    }
  }
}
```

```js
ctx.__('model.user.name');
ctx.__('model.user.createdAt');
```

The before:

```js
{
  'model.user.name': 'Real Name',
  'model.user.createdAt', 'Joined At'
}
```

Benchmarks:

```
Deeps:  9
  2 tests completed.

  direct read a key x 85,993,593 ops/sec ±1.89% (96 runs sampled)
  by nested         x  4,203,837 ops/sec ±0.98% (93 runs sampled)
```

index.js

@@ -103,7 +103,8 @@ module.exports = function (app, options) {
 
     var locale = this.__getLocale();
     var resource = resources[locale] || {};
-    var text = resource[key] || key;
+
+    var text = resource[key] || getNestedValue(resource, key) || key;
     debug('%s: %j => %j', locale, key, text);
     if (!text) {
       return '';
@@ -209,4 +210,13 @@ module.exports = function (app, options) {
     // support zh_CN, en_US => zh-CN, en-US
     return locale.replace('_', '-').toLowerCase();
   }
+
+   // fetch nested key, example: model.user.fields.title
+  function getNestedValue(data, key) {
+    var keys = key.split('.');
+    for (var i = 0; typeof data === 'object' && i < keys.length; i++) {
+      data = data[keys[i]];
+    }
+    return data;
+  }
 };