mod_rest: Roundtrip disco and items when string or boolean is used E.g. {disco:true} turns to <query/> and back to {disco:true}, same with {items:true}. Similarily, {disco:"node"}, {items:"node"} turn into <query node="node"/> and back.

This module adds a command to `prosodyctl` for listing users.

``` sh
prosodyctl mod_listusers
```