Add 'HOOK_FEED_BASIC_INFO' to enable plugins to provide basic feed info.

Add 'HOOK_FEED_BASIC_INFO' to enable plugins to provide basic feed info.

It's expected the plugin will return content parsable by FeedParser, which
will act as an interface to the basic feed info.  In the case of a plugin
that also uses 'HOOK_FETCH_FEED', both might return the same content.

The hook signature was made somewhat similar to 'HOOK_FETCH_FEED'.