Change behavior of before/after and deprecate specific.

[JohnAD]

From @GULPF https://github.com/dom96/jester/issues/226#issuecomment-633048738:

If I understand correctly, the only semantic difference between specific: and before: is that before: will run for all request paths even if it's placed in a subrouter. The behavior of before: is confusing imo and doesn't seem useful (if I wanted a global before: I would put it in the top router). Could the behavior of before: (and after: as well) be changed so that it works as expected inside subrouters, meaning that specific: would no longer be needed?