-i and -n combined

[basxto]

It would be great if superfamiconv tiles allowed to use -i and -n at the same time.

That would allow to build a tileset based on what is given with -n and add all missing tiles from -i

This would allow:

• to build one tileset for multiple images (some might share a few tiles)
• build a tileset with multiple modes (gb and gbc) or different setting (not mirrored and mirrored) if they use the same native tile format