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>
Merge branch 'digest'

* digest:
  feed: Add the digest-post-process setting
  feed: Add the digest setting for multi-entry email

Signed-off-by: W. Trevor King <wking@tremily.us>
CHANGELOG: Mention the digest addition

Documenting:
92c0e76 Merge branch 'digest'

Signed-off-by: W. Trevor King <wking@tremily.us>
Bump to version 3.5

Signed-off-by: W. Trevor King <wking@tremily.us>
Fixed syntactical error when generating HTML mails

Signed-off-by: Dennis Keitzel <github@pinshot.net>
CHANGELOG: Document HTML syntax fix

Documenting:
4aa7f1d Fixed syntactical error when generating HTML mails

The commit by Dennis Keitzel fixed a typo in the HTML which dated back
to 00e2eecc (Spread cmd_run() logic out into Feed methods,
2012-10-04).

Signed-off-by: W. Trevor King <wking@tremily.us>
test/test.py: Update clean_result() to normalize user agents

Catch up with the USER_AGENT changes from 3f9adb5 (feed: Add the
digest setting for multi-entry email, 2013-04-13).

Signed-off-by: W. Trevor King <wking@tremily.us>
test/gmane/2.expected: Remove some whitespace

I'm not sure which version of html2text I used to generate the initial
expected results, but this version was generated with html2text
3.200.3.

Signed-off-by: W. Trevor King <wking@tremily.us>
test/gmane/3: Add tests for HTML generation

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