Add typescript definitions

[HoldYourWaffle]

I added type definitions so people can use this module in TypeScript without any hassle.
I could submit them to DefinitelyTyped but there are some disadvantages, including (but not limited to):

• It being giant mono-repo
• Definition admission & maintenance is really slow
• Definitions can easily get out of sync*
• Requires another package install (which is completely unnecessary and inconvenient)

I hope you're willing to add these definitions, it'd make my life a lot easier 😄

• If you're concerned about maintenance I'll gladly maintain these definitions for you. Just mention me whenever you make a relevant change 😉

[nikitaeverywhere]

@HoldYourWaffle, one question I have though is

Does it matter in which directory to put index.d.ts? I see usually people put it just near the source file, in the same directory. You've decided to put it under types.

[HoldYourWaffle]

Not really, as long as the path in package.json is correct.
I also added a test for the definitions which requires some other files (like tsconfig), I figured putting these in a separate directory would cause the least clutter.

[nikitaeverywhere]

Good! Okay, now it's live as v1.0.2. Thank you for your effort in improving the package!

[HoldYourWaffle]

No problem, feel free to mention me if the definitions need to be updated :)