Send data in base64 format

Raw data sometimes include invalid UTF-8 bytes and that brings errors to
WebSocket clients. To avoid the errors, encode data into base64 before
sending it.

resources/gotty.js

@@ -53,7 +53,7 @@
             data = event.data.slice(1);
             switch(event.data[0]) {
             case '0':
-                term.io.writeUTF16(data);
+                term.io.writeUTF8(window.atob(data));
                 break;
             case '1':
                 // pong