Support Spanner schema and Skylark files.

Adds support for the .sdl file extension which is used for Spanner's
Schema Definition Language. It uses -- for comments in the manner of
.sql files.

Adds support for the .bzl file extension which is used for Bazel's
Starlark language. It uses # for comments in the manner of Python.

testdata/expected/file.bzl

@@ -0,0 +1,19 @@
+# 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.
+
+# Say hello
+def hello():
+  print("Hello world!")
+
+hello()