Use Go's archive/zip to unzip repository archives

Created by: mrnugget

This fixes #109 (closed) by using Go's archive/zip package to unzip files.

(The code is taken from here and slightly modified: https://golangcode.com/unzip-files-in-go/)

It fixes the "interactive" problem, because it silently ignores case-sensitive naming conflicts, which is what you get when using macOS' Finder's unzip utility and unzip on the CLI. "Silently ignores" sounds bad but I think it's what you expect to happen on macOS.

It also handles the chinese characters correctly.

@eseliger Can you test this with your repositories in #109 (closed)?