Support repeated boolean flag options captured in multi-valued fields

Currently, a boolean flag option (arity=0) cannot be defined as an array.

We want to be able to define options like this:
```
@Option(names = "-v")
boolean[] verbose;
```

Then, if the user specifies the verbose flag multiple times, multiple `true` values are added to the array:

```
<command> -v -v -v -vvv

assert verbose.length == 6
```