Option 'captcha_host' is now deprecated in favor of 'captcha_url'.
However, it's not replaced automatically at startup, i.e. both options
are supported with 'captcha_url' being the preferred one.
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.
