add a destroy() function to debug instances

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

src/debug.js

@@ -119,6 +119,7 @@ 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) {
@@ -130,6 +131,11 @@ function createDebug(namespace) {
   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.