Now that 00af718a made the lua read_options behavior much more similar
to the js behavior, the main difference was that lua does not re-read
the config file at on_update (but it does re-apply its stored content)
while js did re-read it.
Now the js on_update also does not re-read the config file and instead
applies its stored original content.
This is slightly hacky by adding an undocumented optional 4th argument
to read_options which allows overriding the config file content.