Add haml -r feature to support view helper feature

Hello,

I think add require feature to gulp-ruby-haml will make generate website more easier and more faster.
I also try use `gulp-ziey-ruby-haml` but it didn't work correctly, due to it modify many codes from gulp-ruby-haml.

But I notice this repo seems no longer update any issue and pull-request, I hope this feature can merge as soon as possible.