Why does _ have word syntax?

Hi,

I present this as a pull request, but really I'd like to know if there is any specific reason why `haml-mode` gives `_` word syntax. This is very confusing for me when I use `forward-word` and `delete-word`, etc.

I've looked through the git history and I see some people being thanked for updates, but I can't quite figure out a reason for this particular line of code.