Add option to mv to remove empty directories

[larkost]

If you s5cmd mv data from a local source with nested directory structure to s3 it leaves all of the directories behind. It would be very helpful to have an option to remove empty directories as it works, so all you would be left with is directory hierarchies that had excluded files in them.

For example if you have:
outer/
to_exclude.txt
inner/
alpha.txt

And ran:
s5cmd mv --remove-empty-dirs --exclude to_exclude.txt outer/ s3://bucket/something

You would wind up only having (locally):
outer/
to_exclude.txt