Update underscore.js to 1.13.6

vendor/underscore/modules/partial.js

@@ -0,0 +1,24 @@
+import restArguments from './restArguments.js';
+import executeBound from './_executeBound.js';
+import _ from './underscore.js';
+
+// Partially apply a function by creating a version that has had some of its
+// arguments pre-filled, without changing its dynamic `this` context. `_` acts
+// as a placeholder by default, allowing any combination of arguments to be
+// pre-filled. Set `_.partial.placeholder` for a custom placeholder argument.
+var partial = restArguments(function(func, boundArgs) {
+  var placeholder = partial.placeholder;
+  var bound = function() {
+    var position = 0, length = boundArgs.length;
+    var args = Array(length);
+    for (var i = 0; i < length; i++) {
+      args[i] = boundArgs[i] === placeholder ? arguments[position++] : boundArgs[i];
+    }
+    while (position < arguments.length) args.push(arguments[position++]);
+    return executeBound(func, bound, this, this, args);
+  };
+  return bound;
+});
+
+partial.placeholder = _;
+export default partial;