Update urls plugin to fetch full text of links to tweets

With this patch, this plugin is capable of recognizing links to specific tweets, and fetching the full tweet text accordingly. For example:


````
             │17:02:43   +chimeracoder | https://twitter.com/chimeracoder/status/334355537095426048       │ 
             │17:02:44        gophrbot | chimeracoder's tweet shows: http://t.co/VfVXPJAefq is finally    │
             │                         | updated! Woohoo! #golang                                         │
````

If fetching the tweet text fails for any reason, the plugin fails gracefully by falling back on fetching the <title> attribute (the existing default behavior).