TypeScript support

[dynastywind]

Hi @GeorgioWan! Thanks for your contribution and I really like this project and have used it to provide some impressjs template for my own usage. Since I'm using TypeScript in almost all my project, I'd be appreciated if you could add TS support in the library when you have time.

[Hank-tku]

Hey! @GeorgioWan!，I'm using this component ,warn me
./node_modules/reactimpressjs/lib/Step.js Module not found: Can't resolve 'react/lib/update',
I have try update react-addons-update, but still warning me. The following my project dependencies:
react@16.9.0
typescript@3.5.3
react-addons-update@15.6.2