Add Flow for static type checking

[thibaudcolas]

I would love to have Flow set up on this project to check the type of code that's not React components. It is already set up on two related projects, the config & way it runs should be fairly similar:

• https://github.com/thibaudcolas/draftjs-filters
• https://github.com/thibaudcolas/draftjs-conductor

Rough tasks:

• Set up flow-bin as a dependency
• Add .flowconfig based on the one in draftjs-filters
• Set up in CI
• Add to Rollup build (strip Flow types)
• Start by adding types (// @flow and the type annotations) to behavior.js or DraftUtils.js.

[thibaudcolas]

Fixed in #178.