gridfinity_item_holder.scad - Add back label support

[jprisant]

Reference: #12

This release removed support for labels. Please add them back. Fwiw, the files are so wildly and wonderfully flexible, might as well keep all the options [instead of removing them] for folks to use [or not] as desired.

abel = "disabled"; // ["disabled", "left", "right", "center", "leftchamber", "rightchamber", "centerchamber"]
// Width of the label in number of units, or zero means full width
label_width = 0;  // .01