Handle boolean array arguments better

This is justified since Boolean types already get a bit of special treatment.

In the case I want multiple levels of verbosity, for example, the following should work:

// ./my-program -vvvv

const arg = require('arg');

const args = arg({
    '-v': [Boolean]
});

console.log(args);
/*
    {
        _: [],
        '-v': 4
    }
*/

Currently, you have to do -v -v -v -v separately, and in turn you get [true, true, true, true], which isn’t exactly elegant.

[pacocoursey]

I have completed this, but I could use a few more use-cases to ensure it serves the correct purpose.

Using your example above, should ./my-program -vvvv -v output '-v': 5?

What if the flag '-vvv' exists? Should the following return case 1 or case 2?

// ./my-program -vvv -vvv

const args = arg({
    '-vvv': Boolean,
    '-v': [Boolean]
});

console.log(args);

/*
    {
        _: [],

        // case 1...
        '-vvv': true,
        '-v': 3

        // or case 2...
        '-vvv': true
    }
*/

Don’t want to miss an edge case 😅

[pacocoursey]

Agreed that returning a number for [Boolean] flags seems strange. Seems like the natural solution is to return an array of booleans [true, true, true, ...] and let the user call .length on it themselves.

The core of the issue still exists though: repeating single-hyphen flags should push additional true values onto that array.