Improve module filtering.

I discovered my [`object.omit`](https://www.npmjs.com/package/object.omit) require was being ignored because the module name contains a dot, so I’ve made a slight change to the filtering function, based on the [module documentation](http://nodejs.org/api/modules.html#modules_file_modules).

I also found that core modules were being, unnecessarily, installed from npm, so I've added [`is-core-module`](https://www.npmjs.com/package/is-core-module) to prevent their inclusion.