Return result in run method of executors

[whotwagner]

Currently the run method does not return any result. Attackm8 should be easy to integrate in other projects. So other software should be able to run executors and work with the result.

requirement for issue #114

[thorinaboenke]

Open questions:

• what should the integration look like
  Import AttackMate and Command,
  1 Attackmate instance: attackmate = Attackmate( <config etc.>)
  Command Instance: shell_command = Command("shell", "echo Hello World")
result = attackmate.run_command(shell_command)

• how do we deal with integrating commands that run in the background? (if supported) --> don't bother with async for now, return None

• how do we deal with integrating inter

Notes: Functionalities focused in the BaseExecutor to make adding executors easier
_exex Function needs return value