[feature] support customization of zsh's compdef?

First line of zsh completion script must be `compdef program_name`. zsh support `compdef -P a_glob_expr`, such as `compdef pip-cache -P pip[0-9.]#` will let this completion script support `pip-cache`, `pip`, `pip3`, `pip3.9`, etc.

## A new feature

```python
    shtab.add_argument_to(parser, compdef="XXXX")
```

Result:
```
#compdef XXX
```