Added 3 new conf options:
* thumb_video_black_retries = 0
* thumb_video_black_retry_offset = 1
* thumb_video_black_max_colors = 4
They control how many retries a thumbnail generation should run before
failure. Each retry seeks the video another N seconds determined by
retry_offset in addition to the original thumb_video_delay setting.
A 'black' thumbnail is detected when the resulting image has max_colors
or less distinct color values in it. This is not exclusive to black, but
attempts to avoid any solid color images.
