[5.2] Register console commands from Kernel

[themsaid]

This PR introduces a method to register console commands:

$this->app['Illuminate\Contracts\Console\Kernel']->registerCommand( Command::class );

One useful use case for this is the ability to register a partial mock of a command to test interactivity:

$command = m::mock('\...\TransCommand[ask]');
$command->shouldReceive('ask')->once()->with('..."')->andReturn('First Name');

$this->app[Kernel::class]->register($command);
$this->artisan('langman:trans', ['key' => 'users.name.first']);