The ornamental design for a coilholder, as shown and described.
FIG. 1 is a top front perspective view of a coil holder showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a reference perspective view showing an environmental coil held by the coil holder.
The broken lines in the Figures show environment which forms no part of the claimed design. The oblique lines in the Figures show transparency and not surface ornamentation.