Add a delegate for cancelling unzips

This adds a zipArchiveShouldUnzipFileAtIndex, which is called before
each call to zipArchiveWillUnzipFileAtIndex, allowing a delegate to
cancel the unzip progress.

Originally I thought this was unnecessary due to https://github.com/soffes/ssziparchive/pull/61 but that one is for zipping, this is for unzipping.