[6.x] Allow adding command arguments and options with objects

[svenluijten]

This PR will allow the end user to define arguments and options in artisan commands by instantiating objects (more specifically, InputArgument and InputOption objects) instead of using "plain" arrays. The benefit of this is that you won't have to refer back to the documentation to see the order in which the array should be constructed. Instead, you can look at the constructor of the InputArgument/InputOption class.

Before:

protected function getArguments() 
{
    return [
        ['name', InputArgument::REQUIRED, 'the argument\'s description', 'default value'],
    ];
}

After:

protected function getArguments() 
{
    return [
        new InputArgument('name', InputArgument::REQUIRED, 'the argument\'s description', 'default value'),
    ];
}

It should be noted that the "old" way still works, this is simply an extra way of doing it.

I've also added a (hopefully useful) test, though I couldn't figure out how to properly "mock" the $application that should be given to the command via ->setLaravel(). I'd appreciate some pointers about how I should go about doing this without whipping up a full Application object.