Log Message:
Update to 3.14. From the changelog:

* New `digest-type` configuration adds optional more widely supported
  `multipart/mixed` format
* New argument `--only-new` on the `add` command to ignore entries in
  feed when added, so only new entries will be sent.
* Fix exception when parsing a feed with a linebreak in its title
* Add a new `subject-format` setting, customise the subject line
* Removed '$' interpolation in config file to allow URLs containing
  dollar signs. Interpolation was not fully supported, and the
  placeholder would not survive a second save. Config files with ${...}
  placeholders will need to be manually edited after upgrading, or a
  save forced by adding and removing a fake feed before upgrading.
* Fix default HTTP User-Agent to use `rss2email` instead of
  `feedparser`, and fix `user-agent` setting to consistently apply to
  both outgoing emails and HTTP requests. This introduces a couple of
  potentially breaking changes:
    * Configurations that came to rely on the `feedparser` UA string
      should adjust to the updated UA string going forward.
    * Configurations that came to rely on the unintended behaviour, that
      configuring the User-Agent header in emails does not affect HTTP
      requests, should use a custom `post-process` hook instead.