Support optional steps

It would be good to have stages that are not mandatory or block another stage from running.

The scenario we have is deploying to different countries that use the exact same code. If a change is made but we only want to deploy to one country, then we can't use Pipeline stages to do the deploy. We have to do a custom stage and deploy from a specific commit, which is extremely messy.

My proposal is that a "optional" param is passed into the stage and the next stage can run without considering the "optional" stage. OR, the entire branch stage has to be triggered manually and the inner stages can be triggered manually in any order.

That way, stages don't have to be "sequential".