Allow passing callable as help epilog

[forman]

I'd like to have the option to also pass a function to the epilog kwarg instead of a str only:

def fetch_epilog() -> str:
    # Here: fetch/load/construct the epilog.
    # Function is called only if "--help" option is used.

@click.command(epilog=fetch_epilog)
def mycli():
    # ...

I don't want the epilog text to be available before help is needed because it

1. takes a little time to construct it (e.g., due to imports of other packages);
2. it occupies RAM (ok, not much, but still) which is only used when --help option is passed.

I'm happy to provide a PR should this feature request be accepted.

[davidism]

I don't plan to add this level of complexity to this feature (and to all the other parts of help formatting as well). The Command class offers many methods that can be overridden to customize how help is constructed. You can override Command.format_epilog in this case.