Add getter/setter for section dividers

[edelarua]

Create new getter (section_div()) and setter (section_div<-()) for section dividers.

[Melkiades]

Thanks, Emily! To do this correctly, it will also be necessary to set something in mf_rinfo to keep pagination correct with the right number of lines (I suspect a bug there due to something not counted). Then getter and application of this happens in toString() after the wrapping