Add support for conditional steps

[JeanMertz]

I haven't thought this through yet, but I've come across some real-world cases that would've benefited from conditional steps.

It would add more complexity to the system, so it's still debatable if the trade-offs are worth it.

As for if we were to do this, the cases I've seen require some way to determine "if the output of step A matches value B, then run this step".

So something like:

steps:
  - name: Step A
    description: ...
    processor:
      printOutput
        output: "hello"

  - name: Step B
    conditional: '{{ output["Step A"] == "hello" }}true{{ endif }}'
    description: ...
    processor:
      printOutput
        output: "world"

Basically, if the conditional field is anything other than a blank string, the step will run. This allows for using the existing templating support (#23) to decide if a step should run.

We'd probably also want to add a Skipped state to the existing step states enum, to make it clear if a step ran or not.

But again, this is very preliminary, we'll see if this really makes things easier, instead of too complex to work with.

To give one example that I came across (with some fictional details):

1. A task takes a variable X, that variable can be a URL to either a post, or a user page
2. if it's a user page, fetch the url to the latest post, then go to step 3, otherwise skip this step
3. fetch the post details