feed: Add the digest-post-process setting

feed: Add the digest-post-process setting

For users that want to manipulate the multi-entry message.  For
example, if the stock `digest for <feed.name>` title doesn't cut it
for you, you can now use a post-processing hook to set the title
however you like.

Signed-off-by: W. Trevor King <wking@tremily.us>