accounts: Support account-notify capability

CHANGELOG.md

 ### Security
 
 ### Added
+* Support for IRCv3 capability [`account-notify`](http://ircv3.net/specs/extensions/account-notify-3.1.html)
 
 ### Changed
 * Casemapping changed from custom unicode mapping to preliminary [rfc7700](https://github.com/ircv3/ircv3-specifications/pull/272) mapping.

irc/accounts.go

 		return false
 	}
 
-	client.Send(nil, server.name, RPL_LOGGEDIN, client.nick, client.nickMaskString, client.account.Name, fmt.Sprintf("You are now logged in as %s", client.account.Name))
-	client.Send(nil, server.name, RPL_SASLSUCCESS, client.nick, "SASL authentication successful")
+	client.successfulSaslAuth()
 	return false
 }
 
 		return false
 	}
 
-	client.Send(nil, server.name, RPL_LOGGEDIN, client.nick, client.nickMaskString, client.account.Name, fmt.Sprintf("You are now logged in as %s", client.account.Name))
-	client.Send(nil, server.name, RPL_SASLSUCCESS, client.nick, "SASL authentication successful")
+	client.successfulSaslAuth()
 	return false
 }
+
+// successfulSaslAuth means that a SASL auth attempt completed successfully, and is used to dispatch messages.
+func (c *Client) successfulSaslAuth() {
+	c.Send(nil, c.server.name, RPL_LOGGEDIN, c.nick, c.nickMaskString, c.account.Name, fmt.Sprintf("You are now logged in as %s", c.account.Name))
+	c.Send(nil, c.server.name, RPL_SASLSUCCESS, c.nick, "SASL authentication successful")
+
+	// dispatch account-notify
+	for friend := range c.Friends(AccountNotify) {
+		friend.Send(nil, c.nickMaskString, "ACCOUNT", c.account.Name)
+	}
+}

irc/capability.go

 
 const (
 	AccountTag      Capability = "account-tag"
+	AccountNotify   Capability = "account-notify"
 	AwayNotify      Capability = "away-notify"
 	ExtendedJoin    Capability = "extended-join"
 	MultiPrefix     Capability = "multi-prefix"
 var (
 	SupportedCapabilities = CapabilitySet{
 		AccountTag:      true,
+		AccountNotify:   true,
 		AwayNotify:      true,
 		ExtendedJoin:    true,
 		MultiPrefix:     true,