Different Atoms for Retry

[rawkode]

Hi,

I'm working on some middleware system and I'd like to use this library for retries.

The middleware already has an API that expects {:ok, _}, {:skip, _}, or {:error, _}

I'd like to use retry, but configure it only to retry on {:retry, _}

Having looked at the code, I don't believe this is possible. Would you accept a PR that made this configurable?

[safwank]

@rawkode Sure. You can pass the list of atoms as an option.

[safwank]

This has been included in v0.9. Thanks for the PR @rawkode.