Hako-zushi r5

In this release:

• When there's multiple candidate tracks in the mkv, Sushi will try to find one marked as default and use it automatically.
• Partial error recovery - when Sushi detects that it started to "go astray", it increases the search window and tries to shift broken segments again. Use the --max-window argument to specify maximum possible window Sushi can use. This new window is used only when --rewind-thresh consecutive broken events are detected (5 by default). This algorithm is highly experimental and might change in the future.
• Various other bugfixes and usability improvements.