A Debug Block

It would be useful to support a print block so that when creating a new test plan you can print out variables e.g. from extractors as you go.  

We could have a debug block which could exist in the step, job or plan level and it takes an array of keys which it will then print out.

debug:
  - `<name>`
  - `<name>`
  - `<name>` 

etc...