Add possibility to execute some steps before and after a test group

[nicolaferraro]

Part of #55, depends on #57

See also #59

Let's add to the yaks-config.yaml file the possibility to run scripts pre and post executing the command:

config:
  namespace: {}
pre:
- script: file.sh
- run: |
    echo Start
post:
- run: |
    echo End
    echo Bye
- run: echo Bye! 

Let's assume linux/osx for the moment (which is also the standard environment in pipelines).

Pre and post are two array of object that can be of type script or run:

• script references an executable file in a path that can be relative to the config file location
• run is basically a sh script that can be executed directly. Under the hood we may materialize it into a real sh file in the os tmp dir and run it using the config file directory as context

Pre are executed before all tests in the test group. Post after.