#37 option for separately compress file contents and directory structure

[Znik]

Currently is no possibility for separately tell to mksquashfs that it should compress file data content, and do not compress directory structure.

Both compression areas are controlled by -noD option indeed.
I think it is useful for performance split -noD option for two different options. For compatibility I suggest leave -noD as is. I think there should be two new options -noT / do not compress directory tree, and -noC / do not compress file data content.