Add userJoinOrUpdate hook

This hook fires on the client side, whenever a user joins or
updates (hence the name). It will pass one thing, the user's info,
into the context. This is mostly just for notification purposes.

src/static/js/pad_userlist.js

@@ -21,6 +21,7 @@
  */
 
 var padutils = require('./pad_utils').padutils;
+var hooks = require('./pluginfw/hooks');
 
 var myUserInfo = {};
 
@@ -529,6 +530,10 @@ var paduserlist = (function()
         return;
       }
 
+      hooks.callAll('userJoinOrUpdate', {
+        userInfo: info
+      });
+
       var userData = {};
       userData.color = typeof info.colorId == "number" ? clientVars.colorPalette[info.colorId] : info.colorId;
       userData.name = info.name;