Move zipfile to peerDependencies

[xamgore]

npm install also installs optional dependencies, zipfile in case of epub package. I suggest moving it to peerDependencies.

At the moment node-gyp is called to compile zipfile on macOS Catalina, Node 13/14, fails, and prints a huge error stack in the log. Every time I call npm install. It can be fixed with npm set optional false, but...