Introduce flag to ignore file extension

I used creek to load an XLSX file which is uploaded to a Rails application. This uploaded file is stored at a temporary location and loses its original file extension. When trying to open the file, creek fails and there is no way to tell creek to ignore the file extension.

This change introduces the option `:check_file_extension => false` when opening a creek book. The default behavior is still the same, i.e. when leaving out the option, it still checks for the file extension.