urlselect: support multiple urls in a message

- use string.gmatch instead of string.match to collect all urls in a
  message

- seek to the end of buffer first before scanning urls. this also
  changed the url numbering so that the most recent one will start at
  index 1.

  storing the most recent urls first solved the issue of the
  same url mentioned more than once and each mentions separated by a lot
  of other urls. previously, you'll have to keep pressing Up until you
  found the first mention of that url.