Add driusan/dkim integration example and tests

This patch adds DKIM signing using https://github.com/driusan/dkim tools
to the example hook.

It also adds an optional integration test to exercise signing and
verification, and corresponding documentation.

docs/dkim.md

@@ -0,0 +1,33 @@
+
+# DKIM integration
+
+[chasquid] supports generating [DKIM] signatures via the [hooks](hooks.md)
+mechanism.
+
+
+## Signing
+
+The example hook in this repository contains an example of integration with
+[driusan/dkim](https://github.com/driusan/dkim) tools, and assumes the
+following:
+
+- The [selector](https://tools.ietf.org/html/rfc6376#section-3.1) for a domain
+  can be found in the file `domains/$DOMAIN/dkim_selector`.
+- The private key to use for signing can be found in the file
+  `certs/$DOMAIN/dkim_privkey.pem`.
+
+Only authenticated email will be signed.
+
+
+## Verification
+
+Verifying signatures is technically supported as well, and can be done in the
+same hook. However, it's not recommended for SMTP servers to reject mail on
+verification failures
+([source 1](https://tools.ietf.org/html/rfc6376#section-6.3),
+[source 2](https://tools.ietf.org/html/rfc7601#section-2.7.1)), so it is not
+included in the example.
+
+
+[chasquid]: https://blitiri.com.ar/p/chasquid
+[DKIM]: https://en.wikipedia.org/wiki/DomainKeys_Identified_Mail