New rule request: Force-Property-Value

[dpagini]

Admittedly, my rule name could use some work.

I have implemented a SitePoint article suggestion for managing z-index with a sass z() function that uses a map. My problem is that I cannot enforce that all my fellow developers use this approach when setting a z-index. This makes me want to go back to using just simple variables, but I do think this approach is pretty compelling, and would hate to change it just because I cannot enforce it's use. I have tried looking to other rules to cover this scenario, but cant seem to find something I can use. It would be nice to say in much the same way I can set property unit restrictions, or force variables, I would like to be able to force the use of a function for this situation.