Add clap cli parser, add threads option and help text

[brasky]

In reference to #20

Not sure about supporting /? which is a windows thing that clap doesn't really work with as far as I can tell. Seems worth supporting since this is a windows tool.

Help text:

> .\target\debug\nmuidi.exe -?
Parallelizes deleting directories which can significantly speed up deleting large deeply nested directories with a large number of files on Windows

Usage: nmuidi [OPTIONS] <DIR>...

Arguments:
  <DIR>...  One or more directories to clean

Options:
  -t, --threads <THREADS>  Number of threads
  -h, --help               Print help
  -V, --version            Print version