configure ejabberd.cfg for diffrent vhost

Initially, 'host_config' is intended for this. However, it does not work with 'modules' and other parts of ejabberd.cfg.

A solution is to start ejabberd, and then modify the modules configuration manually using the Web Admin. However, this is not practical because those changes are not kept after a server restart.

So, I wrote a patch that allows you to define modules in a modular way: New option host_config_add.

With this patch, if you want both vhosts to have same modules, and each vhost to have certain modules:

% This ejabberd server has two vhosts:
{hosts, ["one.example.org", "two.example.org"]}.

% Configuration of modules that are common on both vhosts
{modules,
 [
  {mod_register,   [{access, register}]},
  {mod_roster,     []},
  {mod_privacy,    []},
  {mod_adhoc,      []},
  {mod_configure,  []},
  {mod_disco,      []},
  {mod_stats,      []},
  {mod_vcard,      []},
  {mod_offline,    []},
  {mod_announce,   [{access, announce}]}, % Depends on mod_adhoc
  {mod_private,    []},
  {mod_irc,        []},
  {mod_pubsub,     [{access_createnode, pubsub_createnode}]},
  {mod_time,       []},
  {mod_last,       []},
  {mod_proxy65,    []},
  {mod_version,    []}
 ]}.

% Add some modules to vhost one:
{host_config, "one.example.org", [{{add, modules}, [
  {mod_http_bind, []},
  {mod_muc,        [{host, "chatrooms.one.example.org"}
                    {access, muc},
		    {access_create, muc},
		    {access_admin, muc_admin}]},
  {mod_logxml,    []}
]}]}.

% Add some modules to vhost two:
{host_config, "two.example.org", [{{add, modules}, [
  {mod_muc,        [{host, "muc.two.example.org"}
                    {access, muc},
		    {access_create, muc},
		    {access_admin, muc_admin}]},
  {mod_echo, [{host, "eeeeecho.two.example.org"}]}
]}]}.