Support for required parameter/argument

Hi there,
Great job with this library so far!

I'm missing `required` parameter functionality. Would it make sense for you to add it ?
I see that we could (in rough words) extend this function

```
pub fn Param(comptime Id: type) type {
return struct {
id: Id,
names: Names = Names{},
takes_value: Values = .none,
};
}
```

with a field `.required: bool = false` and extend Errors with something like this:
```
pub const Error = error{
MissingValue,
InvalidArgument,
DoesntTakeValue,
MissingRequiredParam
};
```

I think I can prepare PR with such change.
This is only proposition of changes. What do you think ?