Add a new separator option for fractional classes for HAML compatability

[bandogora]

The / in the width/height fractional classes are not valid for HAML classes; i.e. .w-full.h-1/2 would not build.

There is a work around, .w-full{ class: 'h-1/2' }, but that's not a very elegant solution and kind of defeats the purpose of using HAML.

It would be nice if there was an option to change the / similar to the already existing separator option for changing the :.

I also found a Stack Overflow question about this same issue.