Add castrepeated extension

[aaronc]

The castrepeated extension changes the generated field type of a repeated field and expects that the whole array generated by a repeated field is castable to this type.

Example from the tests:

message CastRepeated {
	repeated ProtoType Field1 = 1 [(gogoproto.castrepeated) = "ProtoTypesPointer"];
}

type ProtoTypesPointer []*ProtoType

// generated struct
type CastRepeated struct {
	Field1               ProtoTypesPointer `protobuf:"bytes,1,rep,name=Field1,castrepeated=ProtoTypesPointer" json:"Field1,omitempty"`
}