Flickr slideshow support

Hi there,

This patch adds support for Flickr slideshows to video_filter. To use it, browse to a set on Flickr, copy the URL and add it as you would any other filter. For example:
[video:http://www.flickr.com/photos/somename/sets/72187602387412629]

You can also copy the URL from the slideshow itself; e.g.;
[video:http://www.flickr.com/photos/somename/sets/72187602387412629/show/]

The filter will pick up on both formats. Trailing slashes are optional.