Allow zip archives for validate command

[klianc09]

I know the recommended way to do things is pushing a folder, but as butler push also supports .zip files (and then just treats them as a folder) I would have expected the butler validate command (https://itch.io/docs/itch/integrating/manifest/validating-your-manifest.html) to also take a zip file as an argument. However when providing a zip file to butler validate it will start in manifest-only validation mode and incorrectly assume the single zip file argument is the manifest (and obviously will fail).

As a workaround I can of course unzip the archive before calling validate, however I would expect validate to work with all arguments that push also works with.