[mod] hardening SearXNG instances by default (formats)

Deny formats has been implemented in 6ed4616d.

To harden SearXNG instances by default, other formats than HTML should be
denied.  Most of JSON, RSS and CSV requests are bots [1]::

    Bots are the only users of this feature on a public instance, and they abuse
    it too much that the engines rate limit pretty quickly the IP address of the
    instance.

[1] https://github.com/searxng/searxng/issues/95



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>