Provide a hook for user-specified inspect functions

lib/sys.js

@@ -32,6 +32,11 @@ var error = exports.error = function (x) {
 exports.inspect = function (obj, showHidden, depth) {
   var seen = [];
   function format(value, recurseTimes) {
+    // Provide a hook for user-specified inspect functions.
+    if (value && typeof value.inspect === 'function') {
+      return value.inspect(recurseTimes);
+    }
+
     // Primitive types cannot have properties
     switch (typeof value) {
       case 'undefined': return 'undefined';