Add support for soju.im/filehost

For now, only handle paste events containing files.

Co-authored-by: Alex McGrath <amk@amk.ie>

lib/irc.js

@@ -526,6 +526,10 @@ export class Isupport {
 		}
 		return parseInt(this.raw.get("LINELEN"), 10);
 	}
+
+	filehost() {
+		return this.raw.get("SOJU.IM/FILEHOST");
+	}
 }
 
 export function getMaxPrivmsgLen(isupport, nick, target) {