pulling Evgeniy Dolzhenko's patch to add _.tap, with tests

underscore.js

@@ -495,6 +495,13 @@
     return typeof obj == 'undefined';
   };
 
+  // Invokes interceptor with the obj, and then returns obj. 
+  // The primary purpose of this method is to "tap into" a method chain, in order to perform operations on intermediate results within the chain.
+  _.tap = function(obj, interceptor) {
+    interceptor(obj);
+    return obj;
+  }
+
   // Define the isArray, isDate, isFunction, isNumber, isRegExp, and isString
   // functions based on their toString identifiers.
   var types = ['Array', 'Date', 'Function', 'Number', 'RegExp', 'String'];