aliases: Implement catch-all

This patch implements support for catch-all aliases, where users can add
a `*: destination` alias. Mails sent to unknown users (or other aliases)
will not be rejected, but sent to the indicated destination instead.

Please see https://github.com/albertito/chasquid/issues/23 and
https://github.com/albertito/chasquid/pull/24 for more discussion and
background.

Thanks to Alex Ellwein (aellwein@github) for the alternative patch and
help with testing; and to ThinkChaos (ThinkChaos@github) for help with
testing.

internal/aliases/testdata/erroring-hook.sh

@@ -0,0 +1,5 @@
+#!/bin/sh
+
+# Hook that always returns error.
+# This could be replaced by /bin/false, but that doesn't work on freebsd.
+exit 1