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/config.go

@@ -27,6 +27,7 @@ type Config struct {
 	ContextType      string                  `yaml:"context_type"`
 	ClientGetter     string                  `yaml:"client_getter"`
 	Bindings         map[string]*TypeBinding `yaml:"bindings"`
+	StructReferences bool                    `yaml:"use_struct_references"`
 
 	// Set to true to use features that aren't fully ready to use.
 	//