account-tag: add proposed extension

Comments welcome, especially on the TODOs:

  - `*` vs empty value vs no value <add>vs no tag?</add>

  - when all three of {WHOX & account-notify & extended-join} are available, should the tag be sent for channel messages or not? *(it is always sent for private messages)*

  - <del>I chose `a` as tag name to avoid wasting space, but I guess I'd be fine with `acct` or `account`.</del> Updated to use `account` as tag name.