feature: allow substituting command_path in command help and epilog text

[Kamilcuk]

Given:

@click.command(help="Example: %(prog)s --do-this")
def cli(): pass

Will result in:

$ ./command --help
Example: ./command --do-this
$ python -m command --help
Example: python -m command --do-this

The problem is that it is not possible to template the command from within the help text itself. There is %(prog)s for version string, but not for help or epilog. It would be nice to have them there, also %(version)s.

Thank you, click is great.