Support for reading / extracting RtMI's .ggpack-files and ggdicts

[JanFrederick00]

Hello, I have modified the program to open Return to Monkey Island's .ggpack*-files and to decode the changes in the ggdict-format. (Note: first time working with go.)

• Implemented the new XOR-Algorithm used by RtMI, keys are extracted from the game's executable file. (2 keys: 256 and 65536 bytes in length)
• RtMI's GGDicts use 16 bits for each string value instead of 32 bit. This change is automatically selected in ggpack (when using the rtmi-key) and selected via a new parameter in ggdict.

The other tools may still need changes, and packing new ggpacks / ggdicts are not yet implemented.

[fzipp]

Thanks for looking into this and for providing an implementation. This works, so I'm going to merge it. I'll probably refactor some parts later, but it's a good starting point.