Backoff constant should be configurable

Currently, if you use `@delay_retry` then the exponent is the constant `10`. This means 5 repeats go from `0.05` -> `0.5` -> `5` -> `50` -> `500`.

If for example, you have an API quota which has 60 second windows, you can't repeat 4 times as you would only cover 55.55 seconds. Instead, you need to go to the next one, which is over 8 minutes long. Being able to set a much smaller exponent would make this infinitely more useful. e.g. if 2 is used:

`0.05` -> `0.1` -> `0.2` -> `0.4` -> `0.8` -> `1.6` -> `3.2` -> `6.4` -> `12.8` -> `25.6` -> `51.2`

This way we only "waste" 42 seconds. Of course, every API and everyone's usage is different, so there many be many even better reasons to make this configurable.