ismindex: Allow adding a path prefix to the generated .ism file

This allows storing the .ismv/.isma/.ismc files separately from
the .ism file on a server, without having to manually edit the
.ism file after generating it with the ismindex tool.

Signed-off-by: Martin Storsjö <martin@martin.st>
ismindex: Add an option for outputting files elsewhere than in the current directory

Signed-off-by: Martin Storsjö <martin@martin.st>