post_process: add documentation and a prettify example

* also mention it in the README file.
* package the filter via setup.py

Signed-off-by: Arun Persaud <apersaud@lbl.gov>
rss2email/post_process/downcase.py: Move my test hook into Arun's directory

All the built-in hooks should live in the same sub-package.  The
`post_process` name Arun used is more descriptive than my `hook`, so
move my downcase code there.

Signed-off-by: W. Trevor King <wking@tremily.us>