This script broke in the migration to Python 3 (see
1101db6a29). This change fixes those bugs,
and also:
1. Allows you to supply multiple source directories. Previously, you
could only provide one, which meant that you'd see lots of unused
strings, even though they were used in a different folder.
2. Adds usage instructions with `--help`, thanks to Python's `argparse`
module.
3. Quits with a `1` status code if any unused strings are found.
4. Prints unused strings instead of used ones.
5. Speeds up the script by only searching for keys. Previously, it'd
also look for comments and blank lines in the `Localizable.strings`
file.
6. Speeds up the script by only reading files once, instead of once per
key.
