feat(parser): support short-option groups

[bcoe]

This is a proposal for supporting short option groups that is quite simple (which I like), and at the same time agrees with an edge-case documented in yargs-parser -- which make me think it's a reasonable approach (I at least haven't noticed many folks complain about this edge-case in the past).

The implementation

Quite simply, -foo expands out to -f, -o, -o, and each individual value is handled by the next step in the parser.

The interesting edge-case of this is that if withValue is configured for the terminal value in the short option list, and the short-option list is followed by a positional, it will be consumed:

const passedArgs = ['-rf', 'foo'];
const passedOptions = { withValue: ['f'] };
const expected = { flags: { r: true, f: true }, values: { r: undefined, f: 'foo' }, positionals: [] };
const args = parseArgs(passedArgs, passedOptions);

As mentioned, this behavior agrees with unit tests in yargs-parser:

// from yargs' test suite:
it('should set the value of the final option in a group to the next supplied value', function () {
    const parse = parser(['-cats', 'meow'])
    parse.should.have.property('c', true)
    parse.should.have.property('a', true)
    parse.should.have.property('t', true)
    parse.should.have.property('s', 'meow')
    parse.should.have.property('_').with.length(0)
})

TODO: document this decision in README.
Fixes #2

[shadowspawn]

Couple of non-blocking suggestions.

[bcoe]

@shadowspawn take a look at the updated FAQ, if you don't mind.

[shadowspawn]

LGTM again.