nth – Generate English ordinal numbers

The com­mand \nth{} gen­er­ates English or­di­nal num­bers of the form 1^st, 2^nd, 3^rd, 4^th, etc. LaTeX pack­age op­tions may spec­ify that the or­di­nal mark be su­per­scripted, and that neg­a­tive num­bers may be treated; Plain TeX users have no ac­cess to pack­age op­tions, so need to re­de­fine macros for these changes.