This extension allows to add a shadow effect to the text contained in a cell. The ShadowCell() method
has the same parameters as Cell(), plus the following ones:
color : color of the shadow. Can be either a string (G for grey,
B for black) or an integer between 0 and 255 (greyscale value). Default value: G.
distance : distance between the shadow and the text. Default value: 0.5.