We claim the ornamental design for a marker holder, as shown and described.
FIG. 1 is a Front-Side Perspective view;
FIG. 2 is a Rear-Side Perspective view;
FIG. 3 is a Left Side view;
FIG. 4 is a Right Side view;
FIG. 5 is a Top view;
FIG. 6 is a Bottom view;
FIG. 7 is a Front view;
FIG. 8 is a Rear view; and,
FIG. 9 is another front-side perspective view.
The broken lines in the drawings are included for the purpose of showing portions of the marker holder and a marker that form no part of the claimed design.