Add support for nesting embeddables

This is a possible approach towards adding support for nesting embeddables (embeddables inside embeddables). I'm not sure whether this implementation is the best solution but I think it is something to start with. It seems to work flawlessly so far but I'm not sure if I missed something, so any feedback is welcome ;)
this is not the only case leading to an infinite nesting. You need to account for loops as well: ``A embed B embed A``