addresses #367

click greatly reworked their completions in pallets/click#1484 and pallets/click#1622.
In consequence, click doesn't add an autocomplete command any more, so I've adjusted the shell snippets required to register completions.

• I've updated README to new mechanism:

  • The autocompletion parameter for options and arguments is renamed to shell_complete. The function must take ctx, param, incomplete, must do matching rather than return all values, and must return a list of strings or a list of CompletionItem. The old name and behavior is deprecated and will be removed in 8.1.

  • The env var values used to start completion have changed order. The shell now comes first, such as {shell}source rather than source{shell}, and is always required.

• I've renamed autoompletion parameters of @click.arguments and @click.options

I've also noticed that #143 is obsolete, as click fully supports fish:
fixes #143