Add query buider 'unless' method (inverse of 'when')

[calebporzio]

Here is a recent snippet of code where "unless" would have been useful:

without "unless"

$query->when($sortColumn !== 'title', function ($query) {
    return $query->orderBy($sortColumn, 'desc');
})

with "unless"

$query->unless($sortColumn === 'title', function ($query) {
    return $query->orderBy($sortColumn, 'desc');
})

I also would like this method in Collection.php - if people like the concept, I'll submit another PR, but let's see how this goes.

Thanks for all the hard work!

• Caleb