Add configurable post-process hooks

On Mon, Jan 21, 2013 at 11:16:58PM -0800, Arun Persaud wrote:
> but I was wondering if there is any chance to add some hooks, so
> that the user can modify the feed before it gets send, something
> that takes the url, uid, and other interesting information and
> returns the body of the feed that should get emailed.

This is not quite what he asked for (e.g., I don't pass the URL
explicitly, the hook should return the full message instead of just
payload, ...), but I think it get's the job done.

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