Add typescript type definitions

[mattbruv]

Would be a nice addition so my IDE doesn't error when trying to use this library.

[movinglinguini]

Hey @mattbruv, sorry it took me so long to get back to you. I'd love to add types but I actually don't know how/what would be the best way to do it to make it convenient for everybody using this. I regularly use Typescript, too, so I feel your pain. Do you have any pointers? Otherwise, feel free to make a PR.