Add null io option

This adds null IO option for efficient handling of IO.
It provides a container directly with `/dev/null` and does not require
any io.Copy within the shim whenever a user does not want the IO of the
container.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

container_test.go

@@ -19,8 +19,7 @@ import (
 )
 
 func empty() IOCreation {
-	null := ioutil.Discard
-	return NewIO(bytes.NewBuffer(nil), null, null)
+	return NullIO
 }
 
 func TestContainerList(t *testing.T) {