On some servers SASL auth would fail because we didn't fully comply with
the spec. Multiple caps can be returned in a `CAP ACK`, and sometimes
servers add a trailing space. Parse these responses more correctly.
Where additional caps were requested using `RequestCaps`, SASL would
send an `AUTHENTICATE` command for each one which servers don't like.
Check that we're responding to the correct `CAP ACK`.
This adds the ability to negotiate arbitrary IRCv3 capabilities on
connection. Capabilities to be requested can be set in `irc.RequestCaps`,
and after registration the negotiated capabilities can be seen in
`irc.AcknowledgedCaps`.
