Test ReactNative about styled-components

[jacobp100]

I could pick this up. Apart from unit tests (we'll probably want to use jest—#325), we might want to do device snapshot tests.

For snapshot tests, we'll need a RN project—should that be in the core styled-components repo, or should we make a new repo for it? I'm inclined to say new repo because it's easier, and because css-to-react-native will also affect the snapshot tests.

I have no idea how to make the CI work either way—if it'll even work with RN 😂

[mxstbr]

We test native now! 🎉 https://github.com/styled-components/styled-components/blob/master/src/native/test/native.test.js