util.stanza: Add method for extracting a single attribute value
Sometimes you only care about a single attribute, but the child tag
itself may be optional, leading to needing `tag and tag.attr.foo` or
`stanza:find("tag@foo")`.
The `:find()` method is fairly complex, so avoiding it for this kind of
simpler use case is a win.
The Prosody project is open to contributions (see HACKERS file), but is
maintained daily by:
- Matthew Wild (mail: matthew [at] prosody.im)
- Waqas Hussain (mail: waqas [at] prosody.im)
- Kim Alvefur (mail: zash [at] prosody.im)
You can reach us collectively by email: developers [at] prosody.im
or in realtime in the Prosody chatroom: prosody@conference.prosody.im