Skip generated files

Ensure it skips for both check only mode and modification mode.

Fixes #39

testdata/expected/file_generated.go

@@ -0,0 +1,7 @@
+// Code generated by "stringer -type foo"; DO NOT EDIT.
+
+package main
+
+func (f Foo) String() string {
+	return "foo"
+}

testdata/initial/file_generated.go

@@ -0,0 +1,7 @@
+// Code generated by "stringer -type foo"; DO NOT EDIT.
+
+package main
+
+func (f Foo) String() string {
+	return "foo"
+}