The ornamental design for a napkinholder, as shown and described.
FIG. 1 is a front, top perspective view of a napkin holder showing a first embodiment of our new design;
FIG. 2 is a front view thereof, the rear view being a mirror image of the front view;
FIG. 3 is a left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a top view thereof;
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a front, top perspective view thereof shown with additional environmental subject matter.
The features shown in broken lines depict surfaces of the napkin holder and environmental subject matter that form no part of the claimed design. The bolded long dash-sho rt dash lines indicate the bounds of the claim and otherwise form no part of the claimed design.