Allow worker to specify which arguments to include in uniquing hash

We're running into an issue where we want to add non-unique arguments to a sidekiq worker, but we'd like the job to still be counted as unique. For instance (pseudocode):

```ruby
MyWorker.enqueue_delayed(product.id, Time.now, 1.hour.from_now)
sleep 10
MyWorker.enqueue_delayed(product.id, Time.now, 1.hour.from_now)
```

Time.now is obviously not going to be unique, and so a second job will be scheduled. What I'd really like is to be able to specify which arguments should be included in the hashing algorithm, so that the second job would not be enqueued.

I'm happy to implement this, but wanted to ensure that it was something that would be accepted as a pull request before doing so.