Add a new feature and fixed a bug

1. Fixed a bug that failed to loading symbol from bundle file.
2. add a option "e" to export "module.exports" of specific JavaScript file.

$ nld a.js -e a.js -o classA.nld

a.js file:
module.exports = function() {};

Making bundle file with "e" option, such bundle file can be used like below:

var ClassA = require('classA.nld');

var a = new ClassA();