rss.c: add suport for feed images (default enabled).

tested with 
./vlc --sub-filter rss --rss-urls "http://www.lefigaro.fr/rss/figaro_une.xml|http://newsrss.bbc.co.uk/rss/newsonline_world_edition/front_page/rss.xml" --rss-length 40 --rss-size 15 --rss-images --rss-speed 1000 <video>