Allow filtering of routed/forwarded packets

MAN page states the following :

    Rules for traffic not destined for the  host  itself  but  instead  for
    traffic  that  should  be  routed/forwarded through the firewall should
    specify the  route  keyword  before  the  rule  (routing  rules  differ
    significantly  from  PF  syntax and instead take into account netfilter
    FORWARD chain conventions). For example:

        ufw route allow in on eth1 out on eth2

This commit introduces a new parameter "route=yes/no" to allow just that.