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/file.c

@@ -1,12 +1,12 @@
 /*
  * 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.