Add hardlink tracking, and an option to disable it

[Freaky]

This adds a simple HashMap-based inode filter, and -l and --count-links flags to disable it, again mirroring BSD and GNU du.

Windows support needs work since it's using unstable features, but the build appears broken there anyway.

[Freaky]

I should probably have mentioned #36 somewhere.

[Freaky]

-% target/debug/dua target
 309.25 MB target
-% du -s --si target
313M    target
-% du -sh target
299M    target
-% target/debug/dua -f MiB target
  294.93 MiB target

Close but not quite.

[Byron]

Fantastic work! I am loving it!

Thanks so much - new release is on the way!