Add option to disable dynamic attributes

Added 2 simple lines so that we can disable dynamic attributes. I saw that it was an earlier issue (#51) that was closed, and you mentioned that pull requests are welcome. We are currently using MtHaml to compile our template files locally into PHP, so this feature is not needed.

We just use Haml because it's nice for us as developers, so in our case MtHaml never needs to hit the production servers thus having to pull it in is an unneeded dependency for our production sites. Have not tested for Twig, but I'm guessing it works the same. This just returns as if the check returned false.

:+1: