Stacking external hybrid modes

From #2072

I think we should come up with a way to stack these.

So we might have a -extern:leet and also an -extern:casemangle. It would be nice to allow:

./john -w=input -extern:leet -extern:casemangle

so that each would would be put through the leet script, and then every word output from the leet script would be put through the casemangle script. This would allow leet to be written 'cleanly', without having to muddy it up doing case mangling also if we wanted to do both leet and casing.

In here, order would be very important. This may also be prone to building duplicate words, so that would need to be looked at also.

Now we 'can' stack them, like this:

./john -w=input -extern:leet -stdout > input2
./john -w=input2 -extern:casemangle

Doing stacking like that may be preferred over trying to stack within 1 run of john. I will wait to hear from others on this. I have NOT started on doing this yet, it may not be trivial so the ROI may not be there.

Start with issue #2072 and the existing -extern mode behavior, including the documented two-command workaround. Determine whether one run should support ordered stacking, then define how ordering and duplicate outputs are handled. Done means the project has a decided approach and its behavior is specified clearly enough to implement and verify.