Add --gen-debug

Uses `-generic_debug` to add `[:log, :statistics]` to all `GenServers`. It can be useful for dev/debug cycle. It will print out some nice info to the shell when a `GenServer` exits abnormally.

I am unsure if this patch should make it in as `-generic_debug` is not documented and only works on `GenServers`, especially when one can do `--erl -generic_debug`.

NB: not tested on windows.