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)
```

benchmark/nested-value.js

@@ -0,0 +1,134 @@
+var Benchmark = require('benchmark');
+var benchmarks = require('beautify-benchmark');
+
+var suite = new Benchmark.Suite();
+
+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;
+}
+
+var resource = {
+  'model.user.foo.bar.aa': 'Hello',
+  model: {
+    user: {
+      fields: {
+        name: 'Real Name',
+        age: 'Age',
+        a: {
+          b: {
+            c: {
+              d: {
+                e: {
+                  f: "fff"
+                }
+              },
+              model: {
+                user: {
+                  fields: {
+                    name: 'Real Name',
+                    age: 'Age',
+                    a: {
+                      b: {
+                        c: {
+                          d: {
+                            e: {
+                              f: "fff"
+                            }
+                          }
+                        }
+                      }
+                    }
+                  }
+                },
+                post: {
+                  fields: {
+                    title: 'Subject'
+                  }
+                }
+              }
+            }
+          }
+        }
+      },
+      model: {
+        user: {
+          fields: {
+            name: 'Real Name',
+            age: 'Age',
+            a: {
+              b: {
+                c: {
+                  d: {
+                    e: {
+                      f: "fff"
+                    }
+                  }
+                }
+              }
+            }
+          }
+        },
+        post: {
+          fields: {
+            title: 'Subject'
+          }
+        }
+      }
+    },
+    post: {
+      fields: {
+        title: 'Subject'
+      }
+    },
+    model: {
+      user: {
+        fields: {
+          name: 'Real Name',
+          age: 'Age',
+          a: {
+            b: {
+              c: {
+                d: {
+                  e: {
+                    f: "fff"
+                  }
+                }
+              }
+            }
+          }
+        }
+      },
+      post: {
+        fields: {
+          title: 'Subject'
+        }
+      }
+    }
+  }
+};
+
+var fullKey = 'model.user.fields.a.b.c.d.e.f';
+
+console.log('Deeps: ', fullKey.split('.').length);
+
+// console.log('getNestedValue:', getNestedValue(resource, fullKey));
+
+suite
+
+.add('direct read a key', function() {
+  resource['model.user.foo.bar.aa'];
+})
+.add('by nested', function() {
+  getNestedValue(resource, fullKey);
+})
+.on('cycle', function(event) {
+  benchmarks.add(event.target);
+})
+.on('complete', function done() {
+  benchmarks.log();
+})
+.run({ async: false });