feed: Add the digest setting for multi-entry email

For high-volume feeds, some users want to receive a single email per
Feed.run() instead of a separate email for each new entry in the feed.
If you enable the new digest setting, the per-entry messages are
packed into a single multipart/digest message instead of being mailed
individually.  The MIME details for digests are spelled out in RFC
2046 [1].

Peripheral changes:
* Added rss2email.feed._USER_AGENT, to get version information into
  the User-Agent message headers and to avoid repeating myself.
* Normalize multipart MIME boundaries for easier testing of
  multipart/digest messages.

[1]: http://tools.ietf.org/html/rfc2046#section-5.1.5

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