Add configuration support for 'tectonic' latex engine

[mikefarmer01]

Tectonic is a modern, user-friendly LaTeX engine (currently in beta state).

I suggest adding support so that Tectonic can be chosen in pandoc-set-latex-engine

[joostkremers]

Thanks, I hadn't noticed that Pandoc added several more pdf engine options. They should now be available.

[mikefarmer01]

Thanks.

Doesn't work for me though.
I enable pandoc-mode, set the PDF engine to "tectonic" using pandoc-set-pdf-engine. Then, I do pandoc-convert-to-pdf.

Result: "pdflatex not found. Please select a different --pdf-engine or install pdflatex".

From the commandline, the following command works though: "pandoc abstract.md -o abstract.pdf --pdf-engine tectonic"

I'm on Windows 10.