TASK: Refactor code

* Reduce indentation.
* Provide internal API to add new feature with necessary checks.

src/Standards/Typo3Update/Feature/Features.php

@@ -47,16 +47,28 @@ class Features implements \Iterator
     {
         foreach (Options::getFeaturesConfiguration() as $featureName => $sniffs) {
             if (in_array(get_class($sniff), $sniffs)) {
+                $this->addFeature($featureName);
+            }
+        }
+    }
+
+    /**
+     * Add the given feature.
+     *
+     * @param string $featureName
+     * @return void
+     */
+    protected function addFeature($featureName)
+    {
         if (!class_implements($featureName, FeatureInterface::class)) {
             throw new \Exception(
                 'Configured Feature "' . $featureName . '" does not implement "' . FeatureInterface::class . '".',
                 1493115488
             );
         }
+
         $this->features[] = $featureName;
     }
-        }
-    }
 
     // implement Iterator interface:
     public function current()