Support Object Instance

[ledongthuc]

Ideally, we support an instance of goretry such as

retry := new goretry.Instance{
   MaxRetries: x,
   MaxWaiting: y,
   Jitter: true,
}

retry.Sequence(...)
retry.Fibonacci(...)

We also support a standard instance goretry.StdInsnace that will be used if the client use retries function directly:

goretry.Std.Sequence(...) === goretry.Sequence()
goretry.Std.Fibonacci(...) === goretry.Fibonacci()