Add describe and epilog fields

As suggested in issue https://github.com/Cofyc/argparse/issues/5, this might not be warranted, but it is a neat feature to have.

Adds the fields `describe` and `epilog` to `struct argparse` and a function to populate these fields. `describe` is printed immediately after the usage and `epilog` is appended in the `--help` text.

Uses the interface described in https://github.com/Cofyc/argparse/issues/5