Add a new option to treat an interface like an object (#97)

## Summary:
The basic idea here is if you only request interface fields (no
fragments) you may not care about the concrete type, and so we could
just generate a struct as if it were an object.  I don't think it's a
good idea to do that by default, because then if you later add a
fragment all your code totally changes, but it's quite reasonable as an
option!

Most of the code involved is just wiring and validation; the
core implementation is literally just: treat it like an object.

Issue: https://github.com/Khan/genqlient/issues/85

## Test plan:
make check


Author: benjaminjkraft

Reviewers: csilvers, StevenACoffman, benjaminjkraft, aberkan, dnerdy, jvoll, mahtabsabet, MiguelCastillo

Required Reviewers: 

Approved By: StevenACoffman

Checks: ⌛ Test (1.17), ⌛ Test (1.16), ⌛ Test (1.15), ⌛ Test (1.14), ⌛ Lint, ⌛ Test (1.17), ⌛ Test (1.16), ⌛ Test (1.15), ⌛ Test (1.14), ⌛ Lint

Pull Request URL: https://github.com/Khan/genqlient/pull/97

generate/generate.go

@@ -209,6 +209,8 @@ func (g *generator) preprocessQueryDocument(doc *ast.QueryDocument) {
 	// needed).  Note this does mean abstract-typed fragments spread into
 	// object-typed scope will *not* have access to `__typename`, but they
 	// indeed don't need it, since we do know the type in that context.
+	// TODO(benkraft): We should omit __typename if you asked for
+	// `# @genqlient(struct: true)`.
 	observers.OnField(func(_ *validator.Walker, field *ast.Field) {
 		// We are interested in a field from the query like
 		//	field { subField ... }