Add sieve pipe and filter (#574)

* First pass at adding support for pipe and filter in Sieve, excluding unit tests.
* Reduce insight into copied scripts
* Added test
* Adjustments related to switch to 16.04

test/config/sieve-pipe/pipe_to_tmp

@@ -0,0 +1,2 @@
+#!/bin/bash
+cat - > /tmp/pipe-test.out

test/config/user2@otherdomain.tld.dovecot.sieve

@@ -0,0 +1,4 @@
+require ["copy", "vnd.dovecot.pipe"];
+if header :contains "subject" "Sieve pipe test message" {
+  pipe :copy "pipe_to_tmp";
+}