Enhancement: Enable alternate help flag

Hi peops,

I just discovered that the `--help` flag does what you'd expect, but `-h` is not a known option.
It's obviously very trivial but might be nice to enable either flag to get help text?

```
$ clef cmip6 -h
Usage: clef cmip6 [OPTIONS] [QUERY]...
Try 'clef cmip6 --help' for help.

Error: no such option: -h
```

cheers
Claire