Support babel-plugin-macros

[ChristianMurphy]

Follow up to #59

Create React App 2 is adding support for babel plugins through babel-plugin-macros facebook/create-react-app#3675
Adding support for babel-plugin-macros would allow usage without ejecting or rewriting create react app.

documentation for creating a macro: https://github.com/kentcdodds/babel-plugin-macros/blob/master/other/docs/author.md

[quantizor]

Closing as this is being implemented in the core library: styled-components/styled-components#2032