Add docs and flexible path to user_config plugin

I needed to make the path modifyable for testing anyway, might as well make it
actually usable.  Not yet tested.

plugins/user_config

@@ -2,10 +2,43 @@
 use strict;
 use warnings;
 
+=head1 NAME
+
+user_config - basic plugin for storing per-user configuration directives
+
+=head1 SYNOPSIS
+
+# in config/plugins
+
+user_config [B<filename pattern>]
+
+=over 4
+
+=item B<config file search pattern>
+
+Pattern to use when searching for user config directory
+Substitute %u for username, %h for host, or %a for full addressn.
+Default: I</home/%u/.qpsmtpd/>
+
+=head1 DESCRIPTION
+
+This plugin implements very basic support for storing user configuration
+in separate directories similar to the global qpsmtpd config directory.
+
+=cut
+
+sub init {
+    my ( $self, $qp, $pattern ) = @_;
+    $self->{pattern} = $pattern || '/home/%u/.qpsmtpd';
+}
+
 sub hook_user_config {
     my ($self,$txn,$user,$conf) = @_;
-    my $username = $user->host or return DECLINED;
-    my $filename = "/home/$username/.qpsmtpd/$conf";
+    my $path = $self->{pattern} or return DECLINED;
+    $path =~ s/%u/$user->user/g;
+    $path =~ s/%h/$user->host/g;
+    $path =~ s/%a/$user->address/g;
+    my $filename = "$path/$conf";
     return DECLINED unless -f $filename;
     my $fh;
     unless (open $fh,$filename) {