More control over delayed extensions

During our migration period from DelayedJob to Sidekiq, we had to use our own fork in order to avoid conflicts with the `delay` method. This allowed us to move sections of the code over to Sidekiq one at a time in production in order to monitor for issues. 

While the delayed extensions are convenient, they are something that is non-essential to the core operation of Sidekiq. It would be incredibly helpful if:

1. Delayed extensions could be disabled on a per-extension basis
2. The method name used for the extensions could be customized to be something besides `delay`

For now, these changes could be backwards compatible so that the delayed extensions are enabled by default. Perhaps in a future release, they could switch to being disabled by default.

If this is something of interest, I can whip up a PR for it.