Add option to disable dynamic attributes

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