Support -propagateInitFuncReturns CLI option

[moznion]

This option is the specifier whether to propagate the returned values come from the init function that is specified through the -init option by the generated constructor.

For example, when it generates the constructor for the following code:

//go:generate gonstructor --type=Struct --constructorTypes=allArgs,builder --init validate --propagateInitFuncReturns
type Struct struct {
	foo string
}

func (s *Struct) validate() error {
	// do something with the created `Struct` value.
	return err
}

then, the generated code becomes like the following:

func NewStruct(foo string) (*Struct, error) {
	r := &Struct{
		foo: foo,
	}

	ret_validate0 := r.validate()

	return r, ret_validate0
}

type Structstruct {
	foo string
}

func NewStruct() *Struct{
	return &Struct{}
}

func (b *Struct) Foo(foo string) *Struct{
	b.foo = foo
	return b
}

func (b *Struct) Build() (*Struct, error) {
	r := &Struct{
		foo: b.foo,
	}
	ret_validate0 := r.validate()
	return r, ret_validate0
}

Ref: #13

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Known Issue

If this option is used with the multiple --type options, probably it won't be the expected result.