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>
Merge branch 'post-process'

* post-process:
  rss2email/post_process/downcase.py: Move my test hook into Arun's directory
  post_process: add documentation and a prettify example
  Add configurable post-process hooks

Signed-off-by: W. Trevor King <wking@tremily.us>
CHANGELOG: Update with summary of post-process addition

Documenting:
3a331d6 Merge branch 'post-process'

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