Revise for middleman 3 1

@j-mcnally This PR moves the logic for registering the Sprockets engine out of the #after_configuration block as otherwise it doesn't work with Middleman 3.1.beta.

I've also added another option to allow you to _not_ ignore the templates folder, if you want to be explicit by ignoring it yourself. The default is still to ignore it.

This works well for me with the Middleman 3.1 branch and still works under 3.0.x as well.