Make startup methods return Results so that the main binary can exit
cleanly instead of using std::process::exit from arbitrary positions,
which exits the process immediately and doesn't run destructors.
Add --manual, --conf-manual command line arguments that display manpages
through a pager. If no pager is found, this currently fails. It should
print the manuals to stdout instead.
The manuals are read from src/manuals and are generated with mandoc
whenever changes to the manpage sources meli.1 and meli.conf.5 are made.