Sync Python and Rust command line format

For now in Python:

usage: ezchlog [-h] [-n] {showconfig,add,list,merge} ...

positional arguments:
  {showconfig,add,list,merge}
                        Choose an action, you can also have more help using ACTION --help

options:
  -h, --help            show this help message and exit
  -n, --dry-run         Dry-run the action

In Rust:

Easy git branch friendly changelogs (ezchlog 0.1.0)
Cyrille Pontvieux <cyrille@enialis.net>

Usage: ezchlog [OPTIONS] <COMMAND>

Commands:
  showconfig  Show configuration
  add         Add a changelog part file
  list        List changelog files
  merge       Merge changelog files into main changelog file under a version
  help        Print this message or the help of the given subcommand(s)

Options:
  -d, --dry-run  Dry-run the action
  -h, --help     Print help
  -V, --version  Print version

It’s similar but different:

• There is no -V/--version in python.
• The --dry-run short version is different in Python -n and in Rust -d
• Commands are badly shown in Python
• There is no description in Python