#2 issue-2 add resolve class for interface

[alexpts]

PR for #2 and nikolaposa/monolog-factory#3

[nikolaposa]

I like the idea with special __class__ key in options, but I think that making this library aware of options as an another special key feels awkward and is not necessary. I would rather keep things as simple as possible, just by introducing that one special argument, so that the class can be constructed this way:

$cascader->create(BufferHandler::class, [
    'handler' => [
        '__class__' => NativeMailHandler::class, //this one gets removed from options after you pick it up
        'to' => 'test@example.com',
        'subject' => 'Test',
        'from' => 'noreply@example.com',
        'level' => Logger::ALERT,
    ],
    'buffer_limit' => 5
]);

[alexpts]

@nikolaposa, I will send commit for it.

[alexpts]

I fixed the comments about flat format