Add 671 RPL_WHOISSECURE to codes

Fixes #50.

Per https://www.alien.net.au/irc/irc2numerics.html, this is used in
KineIRCd as 671 RPL_WHOISSECURE and doesn't have a conflicting
definition. Unreal also seems to use this numeric for this command type:
https://github.com/unrealircd/unrealircd/blob/unreal40/include/numeric.h#L372

Accordingly, just add the code.

Author: Throne3d

lib/codes.js

@@ -529,6 +529,10 @@ module.exports = {
         name: 'err_usersdontmatch',
         type: 'error'
     },
+    671: {
+        name: 'rpl_whoissecure',
+        type: 'reply'
+    },
     900: {
         name: 'rpl_loggedin',
         type: 'reply'