new syslog option

Add syslog option for nxtool (works fine with syslog-ng)

example of syslog-ng.conf :
filter f_naxsi { match("NAXSI_FMT" value("MESSAGE")); };
source WWW.MONSITE.FR {
file("/var/log/nginx/logs/WWW.MONSITE.FR-error.log" flags(no-parse));
};
destination nxtool {
        tcp("10.10.10.1" port(51401) template("$MSG\n"));
};
log { source(WWW.MONSITE.FR); filter(f_naxsi); destination(nxtool); };
ideally : configured from nxapi.json ;)