Includes user's email in profile if returned from facebook API

If the Facebook profile API returns the user's email address, which it does for Workplace integrations, the Botkit profile should include it.

lib/Facebook.js

@@ -254,7 +254,7 @@ function Facebookbot(configuration) {
                         first_name: identity.first_name,
                         last_name: identity.last_name,
                         full_name: identity.first_name + ' ' + identity.last_name,
-                        email: null, // no source for this info
+                        email: identity.email || null,
                         gender: identity.gender,
                         timezone_offset: identity.timezone,
                     };
@@ -765,7 +765,7 @@ function Facebookbot(configuration) {
 
     var user_profile = function(uid, fields, cb) {
         if (!fields) {
-            fields = 'first_name,last_name,timezone,gender,locale';
+            fields = 'first_name,last_name,timezone,gender,locale,email';
         }
         return new Promise(function(resolve, reject) {
             var uri = 'https://' + api_host + '/v2.6/' + uid + '?fields=' + fields + '&access_token=' + configuration.access_token;