Added enclose option in the Uuglifyjs plugin

Added support for the `enclose` option of UglifyJs. Useful way to get even higher compression rates!
Supports receiving objects and arrays;

````js
new webpack.optimize.UglifyJsPlugin({
   enclose: {
      'window':'window',
      'document':'document'
   }
});
````

````js
new webpack.optimize.UglifyJsPlugin({
    enclose: ['window:window', 'document:document']
});
````