Generate random usernames and passwords with random lengths (per account), and write to username.txt and password.txt until each file reaches target size. Usage examples:

	# python3 gen_userpass.py --target-kb 500 --prefix "P@$sw0rD"
	
	# python3 gen_userpass.py --target-kb 500 --min-userlen 4 --max-userlen 8 --min-passlen 4 --max-passlen 8