Refactor into a 'composite' action, much more lightweight now

Changed

• Refactored into a 'composite' action which has following implications ([#4]):
  • Action runs slightly faster because there's no 'node' process in between (or io stream redirects).
  • Action has now just single script input, and you cannot "add" outputs other than automatic "result" and "error".

Removed

• All optional inputs - until "composite" refactor, they were used to pass workflow contexts to the action.
  It's no longer necessary, since 'composite' action can "grab" them on it's own.
• Ability to set custom outputs from the script - now only result and error are set (as outlined in readme).