Support ES6 imports

Tried this:

``` javascript
import detective from 'detective';
import fs from 'fs';

let src = fs.readFileSync(__filename);
let requires = detective(src, { parse: { ecmaVersion: 6, sourceType: 'module' } });

console.log(requires);
```

Prints out `[]` instead of `['detective', 'fs']`. Could we make detective support ES6 imports?