YoutubeIE: report warnings instead of errors if the subtitles are not found (related #901)

For example when downloading a playlist some videos may not have subtitles but the download shouldn't stop.
Change metavar of "--sub-format" from LANG to FORMAT
GenericIE: look for Open Graph info

Only if there is a direct link to the file, don't try if it points to a Flash player
Split FileDownloader in two classes: FileDownloader and YoutubeDL

YoutubeDL is the class that coordinates everything
FileDownloader gets a filename and an info dict and downloads the video.