[rss] Allow enabling/disabling of feeds, add "toggle", remove "clear"

- Added a new "enabled" column in the table.

- Feeds can now be disabled using the toggle command, in which case they
will not fetch new articles.

- The toggle command can take a channel or a feed name or both as
arguments.

- This has also been applied to the "del" command, making the "clear"
command redundant.

- Also, because of the above, feed names have been restricted to
alphanumeric and underscore characters to remove any ambiguity with
channel names.