Fix webpack#1983 : Object properties can be mangle now.

Use the following config:

```js
{
  plugins: [
    new webpack.optimize.UglifyJsPlugin({
        mangle: {
          props: {
            regex: /_$/
          },
        },
      }),
  ]
}
```

Author: hh9527

lib/optimize/UglifyJsPlugin.js

@@ -91,6 +91,9 @@ UglifyJsPlugin.prototype.apply = function(compiler) {
 						ast.figure_out_scope();
 						ast.compute_char_frequency(options.mangle || {});
 						ast.mangle_names(options.mangle || {});
+						if(options.mangle.props) {
+							uglify.mangle_properties(ast, options.mangle.props);
+						}
 					}
 					var output = {};
 					output.comments = Object.prototype.hasOwnProperty.call(options, "comments") ? options.comments : /^\**!|@preserve|@license/;