[Enhancement] Extend flow command with placeholder

[breadlesscode]

Hi and thanks for this great package!

What do you think about adding a placeholder for the flow command execution. Something like this:

Sitegeist:
  MagicWand:
    flowCommand: '<FLOW_CONTEXT> ./flow <FLOW_COMMAND>'

If you are using Docker you can tweak your command for running in a container. I think that would be simple to achrive and really useful too :)