Add centered subheader option

[alexisnsmith94]

Is it possible to center a subheader line under specified header lines? In my example, I would like "Events / N" to be centered under Treatment 1 N and Treatment 2 N? I tried adding white space but grid seems to delete that.

[gforge]

I think this would need some tinkering. The biggest problem is the size of the API - if it becomes too complex then it is impossible to use the package