Write output from `sh' task action directly:

* Accept output as argument in Remote#execute;
* Modify ShellCommand so that env output is directly handled.

lib/producer/core/actions/shell_command.rb

@@ -3,7 +3,8 @@ module Producer
     module Actions
       class ShellCommand < Action
         def apply
-          output.puts remote.execute(arguments.first)
+          remote.execute(arguments.first, output)
+          output.puts
         end
       end
     end