Query structures to support:
- arguments
- repeated fields
- interfaces

Config options:
- schema location (http or file)
- use http client or not
- use ctx or not, maybe
- target url (const or maybe allow it to be dynamic at runtime?
- auth (maybe via client + roundtripper)
- HTTP calling convention???

Misc:
- replace __ with some unicode garbage
- handle graphql errors in a reasonable way