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.