pastebuf: added **open-recent-url action

- you can call `/pastebuf **open-recent-url` inside a buffer to scan any
  pastebin urls and then open it. it only scan 1 url by default. to scan
  more, just specify a number in the third argument.

- detect language of a paste from the query string for sprunge.us and
  vpaste.net

- help++ and readme++