Add variadic RequestOption to Query and Mutate

[iheanyi]

Fixes https://github.com/shurcooL/graphql/issues/28.

I saw various solutions in https://github.com/shurcooL/graphql/issues/28, especially this one: https://github.com/shurcooL/graphql/issues/28#issuecomment-485226864. I decided to take the variadic options approach because:

1. It's backwards compatible
2. Easy to build on top of and set other nice options that users may want to use.

I've gone ahead and included a WithRequestHeader option that users will most likely want.

[iheanyi]

@dmitshur Wanted to follow-up on this, any chance this can get a lookover?

[superbeeny]

bump

[andig]

Also see https://github.com/shurcooL/graphql/pull/73