add channel autojoin feature

See discussion on #2077

default.yaml

     # in a "closed-loop" system where you control the server and all the clients,
     # you may want to increase the maximum (non-tag) length of an IRC line from
     # the default value of 512. DO NOT change this on a public server:
-    # max-line-len: 512
+    #max-line-len: 512
 
     # send all 0's as the LUSERS (user counts) output to non-operators; potentially useful
     # if you don't want to publicize how popular the server is
     # (0 or omit for no expiration):
     invite-expiration: 24h
 
+    # channels that new clients will automatically join. this should be used with
+    # caution, since traditional IRC users will likely view it as an antifeature.
+    # it may be useful in small community networks that have a single "primary" channel:
+    #auto-join:
+    #    - "#lounge"
+
 # operator classes:
 # an operator has a single "class" (defining a privilege level), which can include
 # multiple "capabilities" (defining privileged actions they can take). all

irc/config.go

 		}
 		ListDelay        time.Duration    `yaml:"list-delay"`
 		InviteExpiration custime.Duration `yaml:"invite-expiration"`
+		AutoJoin         []string         `yaml:"auto-join"`
 	}
 
 	OperClasses map[string]*OperClassConfig `yaml:"oper-classes"`

irc/server.go

 	}
 
 	server.playRegistrationBurst(session)
+
+	if len(config.Channels.AutoJoin) > 0 {
+		// only applicable to new clients, not reattaches:
+		server.handleAutojoins(session, config.Channels.AutoJoin)
+	}
+
 	return false
 }
 
 	rb.Add(nil, server.name, RPL_ENDOFMOTD, client.nick, client.t("End of MOTD command"))
 }
 
+func (server *Server) handleAutojoins(session *Session, channelNames []string) {
+	rb := NewResponseBuffer(session)
+	for _, chname := range channelNames {
+		server.channels.Join(session.client, chname, "", false, rb)
+	}
+	rb.Send(true)
+}
+
 func (client *Client) whoisChannelsNames(target *Client, multiPrefix bool, hasPrivs bool) []string {
 	var chstrs []string
 	targetInvis := target.HasMode(modes.Invisible)

traditional.yaml

     # in a "closed-loop" system where you control the server and all the clients,
     # you may want to increase the maximum (non-tag) length of an IRC line from
     # the default value of 512. DO NOT change this on a public server:
-    # max-line-len: 512
+    #max-line-len: 512
 
     # send all 0's as the LUSERS (user counts) output to non-operators; potentially useful
     # if you don't want to publicize how popular the server is
     # (0 or omit for no expiration):
     invite-expiration: 24h
 
+    # channels that new clients will automatically join. this should be used with
+    # caution, since traditional IRC users will likely view it as an antifeature.
+    # it may be useful in small community networks that have a single "primary" channel:
+    #auto-join:
+    #    - "#lounge"
+
 # operator classes:
 # an operator has a single "class" (defining a privilege level), which can include
 # multiple "capabilities" (defining privileged actions they can take). all