The ornamental design for a spongeholder, as shown and described.
FIG. 1 is a front perspective view of a sponge holder design showing my new design.
FIG. 2 is a rear perspective view of a sponge holder.
FIG. 3 is a front view of a sponge holder.
FIG. 4 is a rear view of a sponge holder.
FIG. 5 is a left side view of a sponge holder.
FIG. 6 is a right side view of a sponge holder.
FIG. 7 is a top view of a sponge holder; and,
FIG. 8 is a bottom view of a sponge holder.
Any shading and cross-hatching are not features of the design but are utilized to illustrate the surface contours of the sponge holder design in the drawings.