Add with() method to Block

[lukasbesch]

Updated from 0.2.1 to 0.4.0 and ran into fatal errors because my blocks don't define a with method.

I propose adding a method to the extendable Block class just to be safe.

[tangrufus]

I think adding abstract public function with(): array to AbstractBlock is a better choice.

Besides, we should have an interface for AbstractBlock.

[codepuncher]

Closed in favour of #38