Add support for mapping all nullable types as pointers (#198)

This implements the approach suggested in
https://github.com/Khan/genqlient/issues/178#issuecomment-1077559343.
See the added documentation for the full behavior.

generate/config.go

@@ -26,6 +26,7 @@ type Config struct {
 	ContextType      string                  `yaml:"context_type"`
 	ClientGetter     string                  `yaml:"client_getter"`
 	Bindings         map[string]*TypeBinding `yaml:"bindings"`
+	Optional         string                  `yaml:"optional"`
 	StructReferences bool                    `yaml:"use_struct_references"`
 	Extensions       bool                    `yaml:"use_extensions"`