Polished a bit more RShell by adding the option to extrude only selected vertices/edges/faces.
The default behavior makes a shell of current whole retopo mesh, but if user wants to make a shell of a part only, it can use now the selectOnly feature. Notice that of the seletced element has a boundary it will automatically create the rim , so islands if are fully selected will have a proper rim after extrusion.
A known issue is the way retopo handles normals because, unlike other rooms, retopo enforce and override faces normals, flipping them and preventing to change them. Is something I will try to modify, but at later stages to avoid breaking legacy functionality.