player: load external subs for uncompressed rar archives

Uncompressed rar archives can be transparently opened, but the filename
the player doesn't have the direct filename (but something starting
with rar://... instead). This will lead to external subtitles not
being loaded.

This doesn't handle multi-volume rar files, but in that cases just use
the --autosub-match=fuzzy option.

Fixes #397 on github.