Start of refactoring prettify.py.

Still needs setup.py, launching wrapper in base folder.
Oops, that needs to go here for namespace reasons.
Note about extending the weechat reader.
Plaintext renderer refactored.
--ignore-{type} arguments for dropping various message types.
Handle all except notices in weechat reader.
Transplanted HTML renderer into module, cleaned up.
Correctly space gutter entries.
Better text for join/part/quit messages.
Correctly space lines when final item is overlong.
Convert main run script into a wrapper launcher.

The package can also be installed normally once setup.py is written,
run_prettify.py is only for running it when you don't wish to install
(for example, if following git HEAD).
Make prettify a package.
Make prettify installable with setuptools/distribute.
That is not how you classify.