This adds a `help` verb (and `--help` option) to the `fonttools` command line tool. Submodules will be listed in the help text if they have an importable `main` function with a docstring, and `main`'s docstring will be used as the one-line description for the help text.
The convention is that sys.exit(...) is called only if a module is run as a script,
and that main() entry points use return statements to report exit codes: 0 (or None)
for successful execution, or any non-zero integer for errors.
E.g. see the console scripts generated when installing with pip.
