The ornamental design for a cardholder, as shown and described.
FIG. 1 is a plan view of a card holder according to the design;
FIG. 2 is a front elevational view thereof; the back elevational view being the same;
FIG. 3 is a bottom view thereof;
FIG. 4 is a left side elevational view thereof; and,
FIG. 5 is a right side elevational view thereof.
The portions of the design and cards shown in broken lines in the drawings are included for the purpose of showing features that form no part of the claimed design.