add ability to selectively load files from archive

[bryab]

In working on my little Godot hobby project, I decided I wanted to allow the user to selectively get files from an S3D. In the current implementation of EqArchive, the entire archive is decompressed.

After trying various ways to add the functionality I wanted to EqArchive, I gave up and just made a new struct for this purpose, as I wanted to move on with my project.

Take a look - I don't think this is an ideal solution (two different structs) but it works for now and I will revisit this later.

4b0e79b