add support for typings

[evanshortiss]

Thanks for this module! I'm using it in a project right now that uses TypeScript and found that it has no type information so I generated some simple definitions.

Typically there are two ways to add types to a project:

1. Create a pkg-name.d.ts and referencing it in package.json (like this PR)
2. Adding an entry to github.com/DefinitelyTyped/DefinitelyTyped

Option 1 is somewhat nicer since you just npm install <pkg-name> and start coding. With option 2 you need to also do npm install @types/<pkg-name>.

Let me know if you'd like to merge this. If not I can add to DefinitelyTyped 👍

[floatdrop]

@evanshortiss thanks for contribution, but I would like keep these modules as small as possible.

With option 2 you need to also do npm install @types/<pkg-name>.

flow-typed provides nice cli to install all definitions for npm modules. Perhaps tsd can help with this task.

[sindresorhus]

@evanshortiss I think submitting to DefinitelyTyped would be better. See sindresorhus/semver-diff#3 (comment) for reasoning. I haven't had the time to answer sindresorhus/ama#439 yet.

[evanshortiss]

@sindresorhus that's cool - I can definitely see this side of the coin for maintainers. Thanks for the response 👍