Re-packing

[martinlindhe]

A somewhat common use case is the ability to:

1. unpack a custom archive
2. make some changes
3. reconstruct the custom archive with the changed files

This is a required step in modding of many games.

feng currently does not implement a repack feature, but it should be straight forward.

In order to re-pack a file, we need a input directory of previously extracted files (named correctly).

Map the input file, and iterate over it, replace the (maybe compressed) data blocks and re-calculate the required fields due to changed data sizes.