Log Message:
Update dovecot2-pigeonhole to 0.4.18 for dovecot2 2.2.28.

v0.4.18 12-04-2017 Stephan Bosch <stephan@rename-it.nl>

+ imapsieve plugin: Implemented the copy_source_after rule action. When this
  is enabled for a mailbox rule, the specified Sieve script is executed for
  the message in the source mailbox during a "COPY" event. This \ 
happens only
  after the Sieve script that is executed for the corresponding message in the
  destination mailbox finishes running successfully.
+ imapsieve plugin: Added non-standard Sieve environment items for the source
  and destination mailbox.
- multiscript: The execution of the discard script had an implicit "keep",
  rather than an implicit "discard".