Selection Sets Support

[Hale-ma]

Hi, I enjoy using this library developing my graphql server. I would like to know is there any support for Selection Sets of query? Define different behavior in a function for query with different Selection Sets may enhance the performance. For instance, a query that only lookup a few field may only need a sample SQL to finish while looking for extra field for this fragment may need to have other logic to calculate the field to return . If I can pass the return field to my function, I can do different logic. However, seems query.FieldFunc("name",func) does not support this.

[kevinmichaelchen]

Seems like related code: argsToJson and parseSelectionSet

https://github.com/samsarahq/thunder/blob/babf75b0cd19d8963b164c284a9e661bcaf80538/graphql/parser.go#L91-L109

[uripre]

+1 for that

[uripre]

Probably related to #200 and #201