Bug in the config

[Zeioth]

Because we are defining options like:

M.on_generate_notification = opts.on_generate_notification or true

If we try to set a option to false, it will default to true. We need a better design.

[Zeioth]

We now implement the local function config.set_default(user_opt, default_opt) to sanitize boolean options.

This is necessary for options which default value is not false. But we apply it to all boolean options now to make more reliable.