aliases: Implement aliases hooks

This patch implements two new hooks: alias-resolve and alias-exists.

They are called during the aliases resolution process, to allow for more
complex integration with other systems, such as storing the aliases in a
database.

See the included documentation for more details.

test/t-04-aliases/alias-exists-hook

@@ -0,0 +1,15 @@
+#!/bin/bash
+
+case "$1" in
+"vicuña@testserver")
+	exit 0
+	;;
+"ñandú@testserver")
+	exit 0
+	;;
+"roto@testserver")
+	exit 0
+	;;
+esac
+
+exit 1