Allow subscription types to be added (#84)

Based on @petecoop's PR at the @Folkloreatelier repo (folkloreinc/laravel-graphql@70707ec).

src/Rebing/GraphQL/GraphQL.php

@@ -46,6 +46,7 @@ public function schema($schema = null)
 
         $schemaQuery = array_get($schema, 'query', []);
         $schemaMutation = array_get($schema, 'mutation', []);
+        $schemaSubscription = array_get($schema, 'subscription', []);
         $schemaTypes = array_get($schema, 'types', []);
 
         //Get the types either from the schema, or the global types.
@@ -71,11 +72,16 @@ public function schema($schema = null)
         $mutation = $this->objectType($schemaMutation, [
             'name' => 'Mutation'
         ]);
+
+        $subscription = $this->objectType($schemaSubscription, [
+            'name' => 'Subscription'
+        ]);
 
         return new Schema([
-            'query'     => $query,
-            'mutation'  => !empty($schemaMutation) ? $mutation : null,
-            'types'     => $types
+            'query'         => $query,
+            'mutation'      => !empty($schemaMutation) ? $mutation : null,
+            'subscription'  => !empty($schemaSubscription) ? $subscription : null,
+            'types'         => $types
         ]);
     }