mod_pubsub_post: Allow specifying item id by a special mapping If `{http://jabber.org/protocol/pubsub}id` exists as an attribute at the top level then it is used, otherwise the previous behavior (.id if string)

This module lets you block connections to any remote servers not on a
whitelist.

``` {.lua}
s2s_whitelist = {
    "example.org",
}
```