[MEDIUM] added the 'block' keyword to the config language

[MEDIUM] added the 'block' keyword to the config language

The new 'block' keyword makes it possible to block a request based on
ACL test results. Block accepts two optional arguments : 'if' <cond>
and 'unless' <cond>.

The request will be blocked with a 403 response if the condition is validated
(if) or if it is not (unless). Do not rely on this one too much, as it's more
of a proof of concept helping in developing other matches.