Input filter specification and merging

[vaclavvanik]

Now it is not possible to use input filter merging on input filters created from specification. Merging saves plenty of time when dealing with many input filters. I suppose extending Factory class to enable merging.

Specification example

$config = [
    'input_filter_specs' => [
         MyInput::class => [
             'foo' => [
             ],
        ],
         MyInput2::class => [
             'merge' => MyInput::class,
             'bar' => [
             ],
        ],
    ],
],

What is your opinion?