Fix issue with 'page not found message': introduce pagesRepository config option and point it to tldr-pages/tldr Github repo

Author: igorshubovych

config.json

@@ -1,4 +1,5 @@
 {
+  "pagesRepository": "tldr-pages/tldr",
   "colors": {
     "text": "green",
     "command-background": "black",

lib/messages.js

@@ -8,5 +8,5 @@ exports.emptyCache = function() {
 exports.notFound = function() {
   return 'Page not found\n' +
   'Try updating with "tldr --update", or submit a pull request to:\n' +
-  'http://github.com/' + config.get().repository;
+  'https://github.com/' + config.get().pagesRepository;
 };