plugin XEP-0060, jp (pubsub/set): publish-options implementation: - publishing options (XEP-0060 §7.1.5) implementation. - jp pubsub/set can specify publish-options using `-f` and `-F` - doc has been updated to explain that - psItemSend and psItemsSend now use serialisation for "extra" - publish-options can be specified in extra['publish-options'] with those methods