Add typings

I have recently started using typescript and found out that there are not typings available for react-count-to on https://github.com/DefinitelyTyped/DefinitelyTyped

I thought it might be a good idea to add them, maybe it will encourage more typescript users to try this library.

What do you think @MicheleBertoli ?

Start by reviewing the react-count-to API and the contribution conventions in DefinitelyTyped at the linked repository. Define typings for the library's public exports and verify that the resulting package gives TypeScript users useful type checking.

Written by the indexing model from the issue text.