feat: support graphviz file extension

testdata/expected/file.gv

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2018 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+graph ethane {
+    C_0 -- H_0;
+    C_0 -- H_1;
+    C_0 -- H_2;
+    C_0 -- C_1;
+    C_1 -- H_3;
+    C_1 -- H_4;
+    C_1 -- H_5;
+} 

testdata/initial/file.gv

@@ -0,0 +1,9 @@
+graph ethane {
+    C_0 -- H_0;
+    C_0 -- H_1;
+    C_0 -- H_2;
+    C_0 -- C_1;
+    C_1 -- H_3;
+    C_1 -- H_4;
+    C_1 -- H_5;
+}