add ability to only retry certain errors

[kilianc]

this comes in handy when working with sockets and streams in general, you want to reattempt and recover as much as possible but fail fast in some cases.

[tim-kos]

Well, the way I do it is like this:

someFunction (err) ->
  if err? && err.error in ["errors", "I", "want", "to", "retry"] && operation.retry(err)
    return

Seems easy enough. I doubt a change to the retry module is necessary for this, as the types of errors will depend on your business/app logic. The retry module does not and should not need to know about them.