I claim the ornamental design for a towel holder, as shown and described.
FIG. 1 is a front perspective view of a towel holder, showing my new design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a left-side elevational view thereof, the right-side elevational view being a mirror image thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a rear elevational view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines showing an animal face cut-out in FIGS. 1 and 2, and a towel in FIG. 1, and a spring in FIG. 5, depict environmental matter and form no part of the claimed design.