Feature/retry add optional interval

[edeustace]

I propose adding an optional interval parameter for async.retry, that will introduce a delay between attempts. I have a scenario where I want to retry a call, but not straight away.

This is a non-breaking change.

interval may be passed in as part of an options object as the first parameter to async.retry:

    async.retry({times: 2, interval: 100}, foo, bar);

The existing function signature remains in place:

    async.retry(2, foo, bar); // same as {times: 2, interval: 0}

[aearly]

Cool, a nice little enhancement. I also prefer this API to your previous PR. Good work! 💯