[rss] Changed some debug messages

- Nothing here is really a critical bug that needs to be more than a
warning, and some are just for info.
- Made error messages a bit more consistent.
- Disable feeds on parse errors or malformed XML.
[rss] Oops, forgot to remove clear from docstring and action list

Removed clear from docstring and action list.

Also prepended an underscore to the RSS methods since they are called
internally from the manage_rss method.
[rss] Added a help function

This function checks the given command against existing _rss methods,
and says the docstring of the method if it matches, or a default message
if it doesn't.
[rss] Replaced the toggle command with enable/disable

Probably anyone using this command will specifically want to enable or
disable a feed or feeds, rather than blindly set them to whatever they
were not.

Also fixed the regex in del and toggle to allow feed names with dashes.
[rss] List function now takes parameters

You can optionally specify a channel and/or feed name, and the list will
be filtered to show only matching feeds.