setOptions after creation and more specific types

[PabloSzx]

When I was using this library (very useful, thank you very much) I realized I had to make a new instance of the test client every time I had to change any option, so I made this change, and it works perfectly, as I tested it in my fork.

Also, I found a couple of easy type safety improvements to be made.

[vitorbal]

Thanks, this makes sense to me! Could you also add some documentation to the README so folks know about this feature? Thanks in advance!

[PabloSzx]

@vitorbal I added documentation, updated dependencies and moved TypeScript to devDependencies.

I also mentioned graphql-tag-ts (module augmentation of graphql-tag) in the documentation since I added support for this library.

[vitorbal]

@PabloSzx thanks for the docs, they look very good!

If it's okay by you, I'd prefer if we keep the typescript section out of this PR, to keep the docs focused on the purpose of the library.

I took a look at graphql-tag-ts though and it looks really cool! I'm going to try it out at work once I have a chance 🙂

[PabloSzx]

@vitorbal done

[vitorbal]

Thank you! LGTM 👍

[vitorbal]

Published as apollo-server-integration-testing@2.3.0. Thanks again for your contribution, @PabloSzx!