Preserve Windows CRLF file endings in output file, closes #202

Leaving `newline` as the default of `None` gives us the desired semantics: When *reading*, all newlines are translated to `\n`. When writing, all newlines are translated to `os.linesep`.

Author: kynan

nbstripout/_nbstripout.py

@@ -592,7 +592,7 @@ def main():
             continue
 
         try:
-            with io.open(filename, 'r+', encoding='utf8', newline='') as f:
+            with io.open(filename, 'r+', encoding='utf8') as f:
                 out = output_stream if args.textconv or args.dry_run else f
                 if process_notebook(
                     input_stream=f, output_stream=out, args=args, extra_keys=extra_keys, filename=filename