Add Typescript Support

[chestercharles]

Proposal
As more projects integrate Typescript, it would be nice if the @synapsestudios/fetch-client provided types so that developers can get type safety and intellisense on the inputs and outputs of the client methods.

What should be typed?

• all client method arguments (including options) and return types.
• the client.addPlugin method. We should also export aClientPlugin type so developers can build against that interface.
• the client itself should have a type. It should take type parameters in such a way that we can properly expose helper methods that the developer expects to be accessible on the client.

How do we do this?
There are a few possible options

1. Re-write the library in TypeScript
2. Create Typescript declaration files that live alongside our JavaScript

Option 1 is the most work, but provides a better guarantee that the types provided by the library are correct. Option 2 is less work, but there is more risk that declared types will not be correct.