`--show-trace` and other diagnostics flags not supported

[timbertson]

Does it make sense to just pass all unknown options though to nix-shell? I guess the cache logic might have to consider the argument set given (in terms of whether it changes the resulting environment), but I assume there's a bunch that would be safe to pass through, e.g:

--show-trace
--verbose
--quiet
--timeout VALUE
--fallback
--option KEY VALUE

[xzfc]

C-n-s needs at least to know how many arguments each option takes. Сonsider nix-shell -p --argstr x --run bash, --run here isn't an option but rather a string argument for --argstr.

[xzfc]

Added a bunch of options in 137390213d0b6cedce5f618f3141e69aeeae797c. Most of them do not invalidate the cache.