Support error.timeout_duration for onFailedAttempt.

[issuefiler]

“Trying again in 32 seconds.”

I propose p-retry provide error.timeout_duration, that is, in how much time the next retry begins, for onFailedAttempt.

This can be achieved by reading this._timeouts.

[issuefiler]

Or error.time_off. I don’t mind the property name.

[MCArth]

p-retry uses https://github.com/tim-kos/node-retry and passes all options through

so e.g. you can pass through

				factor: 1.5,
				minTimeout: 500,

as options to p-retry which will result in an initial gap of 500ms between retries with a 1.5x exponential increase factor