mod_pubsub_mqtt: Support atom_title payload type This commit adds the ability to publish and subscribe with arbitrary payload types. It has a breaking change, which is that topics are now of the form: HOST/TYPE/NODE Currently supported types are utf8_data, json and atom_title.

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

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