Doesn't use `@flow strict`, causes errors

### Are you submitting a **bug report** or a **feature request**?

Feature Request
### What is the current behavior?

React-Final-Form doesn't use `@flow strict`, causing issues with projects that have the flow rule `nonstrict-import`. It would be great if RFF could use strict import for flow.

### What is the expected behavior?
For React-Final-Form to be defined strictly, according to flow.

### Sandbox Link

### What's your environment?
react: ^16.8.0,
react-final-form: ^6.3.0,
final-form: ^4.16.1,
node v10.15.3

### Other information
I haven't added much information, just because most of it is self explanatory.
Thanks.