Implement basic `sh' task action feature:

* Executes a command on the remote host;
* Forwards remote standard output.

lib/producer/core/actions/shell_command.rb

@@ -0,0 +1,11 @@
+module Producer
+  module Core
+    module Actions
+      class ShellCommand < Action
+        def apply
+          env.output env.remote.execute(arguments.first)
+        end
+      end
+    end
+  end
+end