Add back "0 + 1*x" prints as "x" functionality

@bpostlethwaite this part is pretty duct tapy.

mathjs can't parse `'+/-'` so here I replace `'+/-'` with the `'mod'` operator which mathjs can parse.

After the calling `toTex()` I replace the generated `\\bmod` string with the TeX +/- sign `\\pm`.