Allow for conditional errors

[doodles526]

Add a set of functions such as

func ConditionalRetry(o Operation, c Conditional, b Backoff) error
func ConditionalRetryNotify(o Operation, c Conditional, n Notify, b Backoff) error

where

func Conditional(e error) bool

The idea being that some operations return errors, and you only want to backoff on certain errors. Otherwise you just want to drop the error through to the receiver of ConditionalRetry or ConditionalRetryNotify

I already implemented a hacky version of this for a project that I needed to have that functionality. If there's interest of supporting this I could clean up what I have and throw up a PR

[cenkalti]

Why not simply wrap the operation function?

Example:

operation := func() error {
    err := realOperation()
    if err == ErrNotThatBad {
        return nil
    }
    return err
}