Add WHOX

[emersion]

This PR documents a common subset of the WHOX extension. The goal here is not to draft a new standard, but document what already exists in the wild. We can extend this spec as a second step if desirable.

"querytype" has been renamed to "token", because it's a much more descriptive name IMHO (and happens to have the same first letter). Let me know if you want to change it back.

Intentionally left out:

• WHOX flags, unimplemented by some servers
• 'd' (distance/hopcount) and 'o' (oplevel), unreliable and not well-defined

References:

• https://github.com/quakenet/snircd/blob/master/doc/readme.who
• http://faerion.sourceforge.net/doc/irc/whox.var
• Ergo impl: implement WHOX ergochat/ergo#1184
• Tentative WHO spec: Specify WHO ircdocs/modern-irc#138

Closes: #81

[emersion]

Any more comments on this spec?

[progval]

LGTM, just one issue.

[jwheare]

Looks ok to merge unless anyone has further comments.