Recursively process files from subdirectories

Currently the script only supports passing a subdirectory as
argument if it also includes some kind of file spec, for example:

  ./pep257.py folder1/file2.py folder2/*.py

This change adds support for passing directory names as arguments
without file spec:

 ./pep257.py file1.py folder

All .py files under the directory will be processed.  Files within
subdirectories will also be processed recursively.

Change-Id: I06befd6a1d2d8de37696e6a9a95a7229a28f3c80