Make "@" and "/" indicate client name and network, respectively

This allows both kinds "<username>@<client>/<network>" and
"<username>/<network>@<client>".

doc/soju.1.scd

@@ -37,6 +37,9 @@ soju supports two connection modes:
   a channel, you need to use the suffix too: _/join #channel/network_. Same
   applies to messages sent to users.
 
+For per-client history to work, clients need to indicate their name. This can
+be done by adding a "@<client>" suffix to the username.
+
 # OPTIONS
 
 *-h, -help*

downstream.go

@@ -540,10 +540,18 @@ func unmarshalUsername(rawUsername string) (username, client, network string) {
 		username = rawUsername[:i]
 	}
 	if j >= 0 {
-		network = rawUsername[j+1:]
+		if rawUsername[j] == '@' {
+			client = rawUsername[j+1:]
+		} else {
+			network = rawUsername[j+1:]
+		}
 	}
 	if i >= 0 && j >= 0 && i < j {
-		client = rawUsername[i+1 : j]
+		if rawUsername[i] == '@' {
+			client = rawUsername[i+1 : j]
+		} else {
+			network = rawUsername[i+1 : j]
+		}
 	}
 
 	return username, client, network