Add tectonic (XeLaTeX) command

It might be worth adding [tectonic](https://github.com/tectonic-typesetting/tectonic) XeLaTeX distribution along with the already available TeX, LaTeX commands/distributions. In my opionion it has several advantages over the more traditional TeX distributions:
* relatively small, more or less just the executable -- the required latex packages are downoloaded on demand. I wonder though whether this might be in violation of the iOS guidelines/rules?
* also replaces latexmk and others, e.g. handles the multiple passes of latex (bibtex, biblatex)