Add UMD plugin format

lib/less/plugin-api.js

@@ -0,0 +1,13 @@
+(function (root, registerPlugin) { 
+if (typeof define === 'function' && define.amd) { define([], registerPlugin);
+} else if (typeof module === 'object' && module.exports) { module.exports = registerPlugin();
+} else { if (!root.less) { root.less = {}; } if (!root.less.plugins) { root.less.plugins = []; }
+root.less.plugins.push(registerPlugin()); } }
+
+(this, function () {
+    return {
+        install: function(less, pluginManager, functions) {
+            // functions.add('')
+        }
+    };
+}));