Rather than using `priority` with `sortProperties`, a new option called
`order` defines the ordering of the sections. I.e.
order = [ "global" "uptime" "banner" ]
means that `uptime` comes before `banner`. Please note that `global` is
for global settings and not a section. I figured that it'd be too much
magic to hide this in the implementation and ask the user to specify the
order of _each_ section in `settings` instead.
OTOH this makes the intent way clearer than priorities. Also, this
remains opt-in, the option defaults to `attrNames cfg.settings`, i.e.
all sections ordered alphabetically.
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