Switch year arg from int to string

This effectively allows for arbitrary copyright
years. For instance, 2018-2019.

testdata/multiyear_file.c

@@ -0,0 +1,12 @@
+/*
+ * Copyright (c) 2015-2017,2019 Google LLC All rights reserved.
+ * Use of this source code is governed by a BSD-style
+ * license that can be found in the LICENSE file.
+ */
+
+#include <stdio.h>
+
+int main() {
+	printf("Hello world\n");
+	return 0;
+}