Add Flow please

Hello react-slingshot team , I'd suggest to add [Flow](https://flow.org/en/) to this project, it may be useful. 😇

Thats all thanks!

The issue names no files or tests. Start by reviewing react-slingshot’s existing Babel, linting, testing, and starter-kit tooling to determine where Flow would integrate. Done means Flow is added with a defined configuration and works across the project’s existing development and testing setup.

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