Using ES6 syntax to cleanup index.js

[huchenme]

index.js is a bit duplicated and unsorted. we can use ES6 syntax to clean up it a bit

from

import add from './add'
export {add}

to

export {default as add} from './add'

[huchenme]

I have created a PR for the proposal as well https://github.com/eggheadio-github/stack-overflow-copy-paste/pull/136

[kentcdodds]

Thanks @huchenme!