Remote thumbnail fetching from trusted sources

Remote thumbnail fetching from trusted sources

So far we only trust i.ytimg.com for YouTube thumbnails, but you can
configure the Oembed plugin in config.php by setting the plugin's class vars:

   addPlugin('Oembed', array('param'=>'value', ...));

Some might think this is a security risk or privacy invasive, but as the Oembed
script is already calling remote sites to get information _about_ linked media,
the way to stop it is to disable the Oembed plugin. However it is not certain
it has been migrated out into a plugin properly yet. But try it if you want to.