The ornamental design for a napkinholder, as shown, and described.
FIG. 1 is a perspective view of a napkin holder showing our new design;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a front elevation view thereof, and the rear elevation view being a mirror image thereof;
FIG. 5 is a right side elevation view thereof, and the left side elevation view being a mirror image thereof; and,
FIG. 6 is a perspective view shown in a use environment.
The broken line seams and hinges in FIGS. 1-6 illustrate the portions of the design that form no part of the claimed design. The broken line napkins in FIG. 6 illustrate environmental subject matter that form no part of the claimed design.