enabled() updates existing debug instances, add destroy() function (

#440)

* dynamically updatable instances

* add a `destroy()` function to debug instances

So that "dynamically created instances" can clean up after themselves

src/debug.js

@@ -13,6 +13,11 @@ exports.enable = enable;
 exports.enabled = enabled;
 exports.humanize = require('ms');
 
+/**
+ * Active `debug` instances.
+ */
+exports.instances = [];
+
 /**
  * The currently active debug mode names, and names to skip.
  */
@@ -114,15 +119,23 @@ function createDebug(namespace) {
   debug.enabled = exports.enabled(namespace);
   debug.useColors = exports.useColors();
   debug.color = selectColor(namespace);
+  debug.destroy = destroy;
 
   // env-specific initialization logic for debug instances
   if ('function' === typeof exports.init) {
     exports.init(debug);
   }
 
+  exports.instances.push(debug);
+
   return debug;
 }
 
+function destroy () {
+  const index = exports.instances.indexOf(this)
+  exports.instances.splice(index, 1)
+}
+
 /**
  * Enables a debug mode by namespaces. This can include modes
  * separated by a colon and wildcards.
@@ -149,6 +162,11 @@ function enable(namespaces) {
       exports.names.push(new RegExp('^' + namespaces + '$'));
     }
   }
+
+  for (var i = 0; i < exports.instances.length; i++) {
+    var instance = exports.instances[i];
+    instance.enabled = exports.enabled(instance.namespace);
+  }
 }
 
 /**