Adds generic parameters to the query/mutations APIs of client

[ctrlplusb]

Doing this allows me to reuse the types generated by GraphQL Code Generator to have a better typed experience via the client instance APIs.

For e.g.

import { createClient } from 'urql';
import { LoginMutation, LoginMutationVariables, LoginDocument } from '../graphql-code-generated';

const urql = createClient({ url: 'http://localhost:1234/graphql' });

urql
  .mutation<LoginMutation, LoginMutationVariables>(
    LoginDocument, 
    { email: 'foo@bar.com', password: 'baz' }
  )
  .toPromise()
  .then(result => {
    return result.data ? result.data.login.email : undefined;
  });

[JoviDeCroock]

This looks good to me personally, great work on that! 👍