Fix for #49

- Define a macro that's essentially a wrapper around \newfontfamily but
  loops over multiple candidates, checking each for availability and
  selecting the first. Suppresses errors from unavailable fonts.

- Use macro to create font-family commands for Calibri/Carlito, and
  KorolevLiU/Calibri/Carlito.

- Also load Carlito for pdfLaTeX if available.