[Feature] Handle async onRetry

[whitecrownclown]

Hello,

This PR is related to #43

I'm not sure about this current implementation, i'd be glad to get someone to review this. (if you don't mind @leo )
A couple of things:

• Name of the flag, right now it's awaitOnRetry;
• Should it have different opts object or be documented separately from node-retry opts ?
• Currently using async await, not sure of the targeted browsers, but i assume promises would be more acceptable, will follow up on this if it's a concern;

Thanks!

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[christiansaiki]

Any chance of getting this merged?
Thanks!