[rss] Lots of new features and refactoring

- RSS functions are now stored in a class, mainly to save the running status without resorting to global variables
- New help command displays each command's docstring
- New fetch command immediately fetches all enabled feeds
- New start and stop commands replace `.startrss` syntax
- List command can now be filtered by channel and/or feed name
- Toggle command has been split into enable/disable
- Debug messages now consistently go to `core.debug_target`
- Feed names can now take dashes
- SQL query strings now use `.format()` instead of deprecated `%` operator