Add zero_override field option

[borosr]

#12
I added zero_override to the field options. This manipulates the getters default return value directory. So based on the example from the ticket, if you are extending the message's field like this:

message Anything {
  string something = 1 [(go_type) = "Something", (go_import) = "", (zero_override)="\"\""];
}

You will get the following result in the related getter:

type Anything struct {
	Something Something

func (x *Anything) GetSomething() Something {
	if x != nil {
		return x.Something
	}
	return ""
}