Removes the need for the plugin to provide feed content.
Gives plugins a chance to provide 'title' and 'site_url' basic info.
Falls back to attempting retrieval+parsing of the fetch URL if needed.
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'.
* add CACHE_MAX_DAYS as a tunable generic expiry interval for various cached files
* add some comments to tunables in functions.php
* rename _MIN_CACHE_FILE_SIZE to MIN_CACHE_FILE_SIZE
* respect MIN_CACHE_FILE_SIZE setting in a few more places where content is cached
