Addresses closed issues #4675, #3896, #1327.
This renders `\subtitle` using the `etoolbox` package if `\subtitle` is not already
defined by the documentclass (as it is in beamer, KOMA, memoir classes).
Based on an example from Enrico Gregorio, <https://tex.stackexchange.com/a/50186>.
Update list of LaTeX packages.
Note that `etoolbox` must be loaded outside the command definition.
Putting it inside causes an error if `\title` is placed after `\begin{document}`.
It's already loaded on LuaLaTeX/XeLaTeX in any case.
fd222b811b