Please release a new version (v0.7.0) that includes the `unique` option.

[HurSungYun]

I’d like to use the unique option for interning in my case, but there is currently no stable tag that includes it. Could you please release a new version with this feature?

message Label {
    string name  = 1 [(vtproto.options).unique = true];
    string value = 2 [(vtproto.options).unique = true];
}