Display disk usage statistics

---

$ du
344   ./build/html/tucs/css
48    ./build/html/tucs/js
1520  ./build/html
400   ./build/linux/examples
696   ./build/linux
2288  ./build
272   ./examples/script
424   ./examples
2840  .

$ du -h      (human-readable)
172K    ./build/html/tucs/css
 24K    ./build/html/tucs/js
760K    ./build/html
200K    ./build/linux/examples
348K    ./build/linux
1.1M    ./build
136K    ./examples/script
212K    ./examples
1.4M    .