Questioning `add_stat_label()`

Couple of things I don't like about this function:
- handling of continuous2 label changes
- what we're doing to the continuous2 label is different from everything else. is this the best place to make these kinds of changes?
- When continuous2 has many rows, it is very annoying to list every single statistic that doesn't change, just to modify one of them. There must be a better API for this.
- we now have functionality in `tbl_summary()` to change the missing row's statistics, but there is no way to change the label for the missing row.

I am not sure if this should be a new function(s), or we can modify the API of this function. But I think changes are warrented.