Add enqueue_jobs option to config

For example, to not enqueue jobs when developing with rails locally:
```ruby
Sidetiq.configure do |config|
  config.enqueue_jobs = !Rails.env.development?
end
```

It still schedules the jobs for easy development, just doesn't send to sidekiq on tick

I made it so that `enqueue_jobs` can be callable, but perhaps that's over the top? (I had to add a simple Config class to do this)