kich clean is slow

```bash
$ time kich clean
real 0m5.366s

# with -maxdepth 5
$ time kich clean
real 0m0.287s
```

The current behavior traverses all of target directory to find broken links.

Ideas to limit maxdepth:
* n - guess for fastest and least correct
* depth($KICH_SRC) - doesn't account for history

Also need a flag (probably `kich clean -f`) to force current behavior.