Added type definitions

[beaumontjonathan]

Added type definitions

Firstly - I love this little library and use all the time! 😀

But I am a big TypeScript user and find myself making a new type declarations file for each new project use it in.

It would be great to merge it into the main repo.

I would be happy to maintain this file should the api ever change.

Please consider this as I believe it adds value to a great library!

Thanks again 😄