Added --subfolder and --subfolder-name commands

[Wohlstand]

These commands can put the whole content of source directory into the sub-directory that will appear on DMG itself. I made these commands a while ago for my own needs, but when I upgraded my copy of script to synchronize with yours, and now I decided to send this argument to mainstream.

To work around, it creates a temporary directory and copies everything into it, and then points a temp directory as a source directory. Just pointing the parent directory will result other unwanted stuff being copied too. As a cost of that, the disk usage will be increased during process of the packing. All temporary data will be removed after process successfully finishes.