[feature] Add "embed" block #488

Having an "embed" block like twig has it, which is basically an "extend" but without an extra file, would be an awesome addition to the library I think.

Is it in the current implementiation possible to get something like this working (or is there maybe even something like this already)?

This would be very useful if a macro is just to small or you want to get more markup replaced in a general structure (like a modal).