Update the regexp for extracting an ID from a tweet.

* twittering-mode.el: Update the regexp for extracting an ID from
a tweet.
(twittering-extract-id-from-url): update the regexp for extracting
an ID from a tweet.
Avoid rendering same tweets repeatedly.

* twittering-mode.el: Avoid rendering same tweets repeatedly. The
function `twittering-render-timeline' rendered given repeated
tweets as is. Since a merged timeline may include one tweet
multiple times, such a tweet was rendered multiple times.
(twittering-render-timeline): skip repeated tweets.