Adding support for delay parameter in retry operator

[starlight173]

I think it would be helpful if this feature is available.

[malcommac]

retry operator now supports delay parameter starting from 2.0.4.
This is an example:

func retry(_ attempts: Int = 3, delay: TimeInterval = 0,
               _ condition: @escaping ((Int, Error) throws -> Bool) = { _,_ in true }) { ... }

By default value is set to 0 (which means no delay) but you can set your own value expressed in seconds.