hooks: Add dkimpy support

This patch adds support in the default hook for using dkimpy for DKIM
signing.

Unfortunately, dkimpy binaries have the same name as driusan/dkim's, so
we need to use --help to disambiguate. It's not pretty but it should
work, and is quite self contained.

Also, for the integration tests, we still need driusan/dkim because
dkimpy lacks the features needed. Specifically, dkimpy's dkimverify
can't be made to use custom DNS, or override the TXT values in any way,
so we can't verify that the generated signature is reasonable.

Thanks to ne9z@github for suggesting this change and providing an
alternative patch in https://github.com/albertito/chasquid/pull/19.

test/t-19-dkimpy/msmtprc

@@ -0,0 +1,14 @@
+account default
+
+host testserver
+port 1587
+
+tls on
+tls_trust_file config/certs/testserver/fullchain.pem
+
+from user@testserver
+
+auth on
+user user@testserver
+password secretpassword
+