Provide TypeScript types to the project

[bryantee]

Would be great if we got some static type checking out of the box. Granted this isn't a massive library, but anything helps. I can try and tackle this if it is of any value.

Somewhat related to #6 for adding Flow support.

[jordanbyron]

@bryantee I don't have a lot of exposure to typescript so I won't be much help in rolling it out. As long as whatever we do is backwards compatible I'm happy to review it. Thanks!

[JonnyBurger]

I installed the update with the TS definition file.

Two questions.. Is userInfo an optional property? Now I get a TS error because I did not specify userInfo property.
Also, must it be of the shape {url: string} or can it be an arbitrary object?

cc @kevinhermawan

[kevin-hyperjump]

@JonnyBurger userInfo is required property and and the value can't an arbitrary object

[JonnyBurger]

@kevinhermawan Hmm, why does the comment say in the README

userInfo: {
   url: "app://orders" // Provide any custom data like deep linking URL
}

?