Release b4 v0.6.0

- Extended trailer format recognition for follow-up trailers, e.g.:
  Reviewed-by: Dev Eloper <dev@eloper.com>
  [for some_foo_function in bar.c]

- Add support for local maildirs, allowing to grab threads straight from
  a local maildir or mailbox, e.g.:
  b4 am -m ~/Mail <some-msgid>
  (don't try this on huge mailboxes, though, as there is no support for
  intelligent header search -- b4 will iterate ALL messages looking for
  the thread you need).

- Preserve trailer order by default instead of sorting them. If you
  like your trailers sorted, define the order in the config file; see
  b4(5) for configuration file details.

- Perform DKIM verification on all patches and messages containing
  follow-up trailers. To turn off, set b4.attestation-check-dkim=no in
  gitconfig. To work, DKIM attestation required dkimpy and dnspython
  libraries; if not found, DKIM checks will not be performed.

- Reimplement developer patch attestation using in-header hashes instead
  of detached certification documents. For more info on the proposal,
  see:
  https://git.kernel.org/pub/scm/linux/kernel/git/mricon/patch-attestation-poc.git/plain/README.rst
  To set up PGP attestation for your patches, see README.rst. It's free!

- A slew of fixes and improvements, many already backported to 0.5.y.
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