tldr-missing-pages: Add support for zsh history file

Missing pages can now also be detected from zsh history files. This is useful is users use zsh instead of bash in their day-to-day work (as I do).

It now also supports reading from compgen -c, which lists all available commands on the system.

If you accept this change, could you please be so kind as to add the hacktoberfest-accepted label to the MR? Thank you.