common.cpp: add --enable-special-out and --disable-special-out for ov…

[mofosyne]

…erride default special token handling behavior

Possible approach to address @HanClinto idea for a flag ggml-org#6923

Investigated using --conversation flag, but it touching multiple other logic so has much different semantic than this idea.

Also @jart would prefer keeping the different default special token behavior.

It would be nice if the 'settings' store has default behaviour + override flag... e.g. DEFAULT, OVERRIDE_TRUE, OVERRIDE_FALSE or something. But anyway this is the best approximation

[jart]

The --conversation flag is so new I honestly don't know what it does.

I don't think it makes sense here to have two flags. I did that before, with --fast and --precise, but I think that's really an exception to the rule. Ideally the project should make a decision about what the default behavior should be, and then have a single flag to make that not happen.