Handle execution of command. The handler may be a class, object, or block (see examples below). === Examples # Simple block handling c.when_called do |args, options| # do something end # Create inst of Something and pass args / options c.when_called MyLib::Command::Something # Create inst of Something and use arbitrary method c.when_called MyLib::Command::Something, :some_method # Pass an object to handle callback (requires method symbol) c.when_called SomeObject, :some_method