graphql: support simple arguments like offset, limit and distinct_on

[kolharsam]

This PR does some clean-up of the code and resolves #12 and resolves #23 too

To test:
Schema: users(id serial primary key, first_name text, last_name text)

{
    limitUsers: users(limit: 2) {
        first_name
        last_name
    }
    distinctUsers: users(distinct_on: last_name) {
        first_name
        last_name
    }
    offsetUsers: users(offset: 3) {
        first_name
        last_name
    }
}

[iykekings]

Code review completed

[iykekings]

Functionality-wise:
This might be sth we didn't catch earlier-on:
When I send this payload

{
    limitUsers: users(limit: 1) {
        first_name
        last_name
    }
}

I get this back with the users' alias changed to lowercase

{
    "data": {
        "limitusers": [
            {
                "first_name": "Ikechukwu",
                "last_name": "Eze"
            }
        ]
    }
}

[iykekings]

Everything works as expected (offset, limit and distinct_on). Great work @kolharsam ❤️‍🔥

[iykekings]

❤️🔥