Fix that #hyperlink? wasn't passing through its sheet, and defer its implementation to #hyperlink, which is equivalent.
Minor refactoring of read_hyperlinks - inline some locals, map the hash, use Roo::Base rather than Roo::GenericSpreadsheet, and scope some work down to the condition in which its needed.
DRY-up Excelx#process_zipfile
Fix that the mode addition of #88 wasn't backwards-compatible with the old options provision.