Typescript support

[matewka]

Have you thought about adding TypeScript support to this library?

I'm working on a fork of your library and I thought it would be nice to rewrite it in TypeScript.

If there are no objections, I'd be willing to add a pull request with it.

[bombillazo]

This would be nice , I agree.

For the time being, you can install npm install -D @types/pixelmatch and they've created typings fro this lib

[mourner]

I added first-class types in #149 (via JSDoc) — would that work?