Add fail2ban filter.

Please put the shadowsocks.conf into your filter.d directory, and
using `filter = shadowsocks` to use the filter.

utils/fail2ban/shadowsocks.conf

@@ -0,0 +1,5 @@
+[Definition]
+
+_daemon = shadowsocks
+
+failregex = ^\s+ERROR\s+can not parse header when handling connection from <HOST>:\d+$