Support tests for flow types

[sobolevn]

We need to tests that types are working. For this specific case there's such a thing as flow type-testing: https://github.com/flowtype/flow-typed/blob/master/definitions/npm/axios_v0.18.x/test_axios_v0.18.x.js

We need to cover existing libs with tests. That's how I see it:
0. We implement a required .flowconfig file with the correct settings (it can be copied from flow-typed

1. The source packages we write tests for are added as a dev-dependencies into the each package that uses this library
2. Then we write flow tests