Limit composer length

Often times IRC servers will truncate messages which are too big.

lib/irc.js

@@ -479,6 +479,36 @@ export class Isupport {
 	bot() {
 		return this.raw.get("BOT");
 	}
+
+	userLen() {
+		if (!this.raw.has("USERLEN")) {
+			return 20;
+		}
+		return parseInt(this.raw.get("USERLEN"), 10);
+	}
+
+	hostLen() {
+		if (!this.raw.has("HOSTLEN")) {
+			return 63;
+		}
+		return parseInt(this.raw.get("HOSTLEN"), 10);
+	}
+
+	lineLen() {
+		if (!this.raw.has("LINELEN")) {
+			return 512;
+		}
+		return parseInt(this.raw.get("LINELEN"), 10);
+	}
+}
+
+export function getMaxPrivmsgLen(isupport, nick, target) {
+	let user = "_".repeat(isupport.userLen());
+	let host = "_".repeat(isupport.hostLen());
+	let prefix = { name: nick, user, host };
+	let msg = { prefix, command: "PRIVMSG", params: [target, ""] };
+	let raw = formatMessage(msg) + "\r\n";
+	return isupport.lineLen() - raw.length;
 }
 
 export const CaseMapping = {