mod_muc_markers: Add option for @id rewriting, default off (may break some clients) XEP-0333 was updated to clarify that stanza-id should be used instead of the 'id' attribute when in a MUC. Some clients still use the id attribute, which is why we were rewriting it. Rewriting is bad because mod_muc advertises stable_id, indicating that Prosody does *not* rewrite ids. Recent versions of Conversations actually depend on this being true. All clients should migrate to using stanza-id for markers. See XEP-0333.

This module publishes data from [Prosody's `internal` statistics][doc:statistics]
 into a PubSub node in [XEP-0039] format.

The node defaults to `stats` but can be changed with the option
`pubsub_stats_node`.

``` lua
Component "pubsub.example.com" "pubsub"
modules_enabled = {
    "pubsub_stats";
}
pubsub_stats_node = "statistics"
```