expose all parameters on the cli to allow unattended use

currently when I run, for example:

```
nix-init -u https://pypi.org/project/some-package
```

I need to manually approve version, builder etc.
Instead I'd like to be able to auto-approve all defaults and optionally override some, for example the following would accept all defaults apart from builder:

```
nix-init -u https://pypi.org/project/some-package -y --builder buildPythonPackage
```
