When the option is set to `true`, the module will generate
CAPTCHA challenges for incoming subscription requests. The option
also implies that option `drop` is set to `true`. Note that
the module won't generate CAPTCHA challenges for messages: they
will still be rejected if `drop` is set to `true`.
Fixes#2246
The callback is supposed to provide known options and their default
values, as long as the documentation. Passing default values into
get_mod functions is now deprecated: all defaults should be provided
by the Mod:mod_options/1 callback.
