Add CLI option `-e` to ignore external modules

This option tells the compiler to ignore external modules, instead of trying to find/parse/validate them.

e.g.

    // bar.js
    import React from 'react';
    export var bar = 'bar';

    // foo.js
    import {bar} from './bar';

When the `-e` option is passed, the compiler will see that the module `'react'` is external and therefore ignore trying to find it, parse it, and validate it.