Add endpoint to obtain logs

It goes through a WebSocket; the secret is sent, and if valid, the logs
are sent back.

lib/log.js

@@ -1,5 +1,7 @@
 'use strict';
 
+const listeners = [];
+
 // Zero-pad a number in a string.
 // eg. 4 becomes 04 but 17 stays 17.
 function pad(string) {
@@ -19,9 +21,23 @@ function date() {
 }
 
 module.exports = function log(...msg) {
-  console.log(date(), ...msg);
+  const d = date();
+  listeners.forEach(f => f(d, ...msg))
+  console.log(d, ...msg);
 };
 
 module.exports.error = function error(...msg) {
-  console.error(date(), ...msg);
+  const d = date();
+  listeners.forEach(f => f(d, ...msg))
+  console.error(d, ...msg);
+};
+
+module.exports.addListener = function addListener(func) {
+  listeners.push(func);
+};
+
+module.exports.removeListener = function removeListener(func) {
+  const index = listeners.indexOf(func);
+  if (index < 0) { return; }
+  listeners.splice(index, 1);
 };