An internal property for use with custom transform types; this property can be used to place all output
in an internal directory, so that a final step in the transform type can do some form of post-processing
before the files are placed in the specified output directory.
For example, if a custom HTML5 transform sets the property to zip_dir, all output
files (including HTML, images, and CSS) will be placed within the directory zip_dir
in the temporary processing directory. A final step can then be used to add more files, zip the directory,
and return that zip to the designated output directory.