Adding Key support and Key Array support to flash messages

[geggleto]

In my apps, I find having different keys to store flash messages a critical component. Being able to flag [warn] or [error] messages for rendering in my UI is pretty critical. This PR attempts to add this functionality.

[gabriel403]

Should $key be a set of constants on the class (or a static array of key => key)?

const INFO = 'info';
public static $messageLevels = [Messages::INFO => 'info', ...];

And the class check that $key is valid before allowing it in (or retrieving it) ?

$flash->addMessage($flash::INFO, 'Toast');

function addMessage($key, $value)
{
    if (! in_array($key, static::$messageLevels)) {
        throw new Exception('sux');
    }
}

Just curious.

[codeguy]

I guess you could have constants, but I'm not sure we need them. Up to you though.

[geggleto]

I think it should be up to the app itself to maintain the set of constants.

[geggleto]

/cc @codeguy @gabriel403 fixed it all up