Fixes#9https://github.com/bootandy/dust/issues/9
Instead of printing the all sub tree leaves only the last leaf is now
printed. See readme change for example
The flag '-p' was added to print the sub tree the old way
Advised to use walkdir by burntsushi as using recursion on file systems
can blow the stack.
walkdir is slower but allows the code to be cleaner and more reliable
Also experimented with ignore but locking the hashmap resulted in
similar performance to walkdir but with much uglier code.
andy.boot
Joe Ardent