Add "struct_references" configuration (#155)

Adds a "struct_references" configuration that will:

* Use a pointer type for struct fields that are a complex type
* The behaviour can be overridden by setting "pointer: false" in the
  proceeding comment blocks
* Sets the "omitempty: true" flag on fields matching the criteria.
  This can also be overriden by setting "omitempty: false"

Although we'd debated not setting the pointer for array elements, it did turn out to be simpler to set them everywhere and also made the documentation cleaner to have a single rule to explain.

Fixes #149.

generate/testdata/snapshots/TestGenerateWithConfig-StructReferences-testdata-queries-generated-structrefs.go

@@ -0,0 +1,59 @@
+package queries
+
+// Code generated by github.com/Khan/genqlient, DO NOT EDIT.
+
+import (
+	"context"
+
+	"github.com/Khan/genqlient/graphql"
+)
+
+// SimpleQueryResponse is returned by SimpleQuery on success.
+type SimpleQueryResponse struct {
+	// user looks up a user by some stuff.
+	//
+	// See UserQueryInput for what stuff is supported.
+	// If query is null, returns the current user.
+	User *SimpleQueryUser `json:"user"`
+}
+
+// GetUser returns SimpleQueryResponse.User, and is useful for accessing the field via an interface.
+func (v *SimpleQueryResponse) GetUser() *SimpleQueryUser { return v.User }
+
+// SimpleQueryUser includes the requested fields of the GraphQL type User.
+// The GraphQL type's documentation follows.
+//
+// A User is a user!
+type SimpleQueryUser struct {
+	// id is the user's ID.
+	//
+	// It is stable, unique, and opaque, like all good IDs.
+	Id string `json:"id"`
+}
+
+// GetId returns SimpleQueryUser.Id, and is useful for accessing the field via an interface.
+func (v *SimpleQueryUser) GetId() string { return v.Id }
+
+func SimpleQuery(
+	ctx context.Context,
+	client graphql.Client,
+) (*SimpleQueryResponse, error) {
+	var err error
+
+	var retval SimpleQueryResponse
+	err = client.MakeRequest(
+		ctx,
+		"SimpleQuery",
+		`
+query SimpleQuery {
+	user {
+		id
+	}
+}
+`,
+		&retval,
+		nil,
+	)
+	return &retval, err
+}
+