Improve file signature

[casey]

filepack archive, added in #80, uses FILEPACK as the archive file signature.

I think the file signature could be improved in a few ways:

• Use an invalid UTF-8 sequence, to make it impossible for a plain text file to start with the file signature.
• Use emoji, since as far as I know, there are no widely used file signatures that contain emoji
• Add a version number
• Make it longer/more random, so it's unlikely to appear by accident