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