Issues with typescript declarations

[JameeKim]

I was looking for a simple graphql client library and chose this one, and tried to use it in my typescript project. But even before I was able to setup the client instance, several problems stopped me.

• All the interfaces are hidden (not exported), so I couldn't use them to declare types of my variables
• As far as I understood by looking into the source code, not all properties in the interceptor objects are required, but the interface FetchQLInterceptor requires them all
• The return types of the methods in the intercepter objects seem very odd; return Promise.reject(error) in the requestError does not work, even though the README has it as an example
• There are some typos, like reueqstError in the interceptor or rejedct in the returned object of FetchQL instance methods (and now I realize they should return promises, I guess?)

If someone else can handle this then it is fine, but maybe I can fix some of the issues if that is needed. But I'm not sure if I will understand all underlying things going on in the source code. I also suggest adding typescript as a dev dependency and then testing if the type declarations enable users to successfully compile. Should I start working on this tomorrow? (It's late at night here so I need to go to bed)

[gucheen]

@JameeKim I've renewed the declaration file in v2.3.0. Please try the new version and check if the declaration file works well.

[gucheen]

PRs are welcome.