Add resolve to src folder

[viankakrisna]

So we can write
import App from 'App'
instead of
import App from './App'

I think this will improve the developer experience of people using this kit :D

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[mxstbr]

This is a webpack-specific thing that people have to rely on in their App code, instead we'd love to use the NODE_PATH env var as that's a more portable solution. (there is an ongoing discussion in #253)

[gaearon]

Yeah, happy to take a solution that works with NODE_PATH instead.