feature: mdox-exec follows certain statements instead of sed by line number.

[bwplotka]

Having code blocks with mdox exec like below is a little bit fragile:

   go mdox-exec="sed -n '24,28p' examples/thanos/e2etest_test.go"
   	// Start isolated environment with given ref.
   	e, err := e2e.New()
   	testutil.Ok(t, err)
   	// Make sure resources (e.g docker containers, network, dir) are cleaned.
   	t.Cleanup(e.Close)
   

Everytime code change it can go off focus. Perhaps there is a way to follow some statements (: