Trim trailing whitespace

If a license text had blank lines, they would result in a trailing
whitespace when prefixed with the line comments.

This change removes trailing whitespace from the middle lines,
leaving top and bottom as is.

Fixes #10

testdata/expected/file1.sh

@@ -1,12 +1,12 @@
 #!/bin/bash
 # Copyright 2016 Google Inc.
-# 
+#
 # 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.