Add support of fragments' arguments

[SudoBobo]

Currently they are not supported by graphql-lua. This will NOT work:

... on Image (size: small) {
    resolution
    uploadedAt
}

It reduces feature capacity of union connections in compare with 'single' ones, as fragments are the only way to access union fields. See graphql/graphql-spec#204

[Totktonada]

I think we can add arguments to 'box field' in order to support arguments for multi-head connections.

[Totktonada]

I'll close it as dupe of more general #185.