Fixes #437 -- add an 'immediate' parameter to _.debounce.

underscore.js

@@ -548,15 +548,17 @@
 
   // Returns a function, that, as long as it continues to be invoked, will not
   // be triggered. The function will be called after it stops being called for
-  // N milliseconds.
-  _.debounce = function(func, wait) {
+  // N milliseconds. If `immediate` is passed, trigger the function on the
+  // leading edge, instead of the trailing.
+  _.debounce = function(func, wait, immediate) {
     var timeout;
     return function() {
       var context = this, args = arguments;
       var later = function() {
         timeout = null;
-        func.apply(context, args);
+        if (!immediate) func.apply(context, args);
       };
+      if (immediate && !timeout) func.apply(context, args);
       clearTimeout(timeout);
       timeout = setTimeout(later, wait);
     };