`mutmut show --all` flag

[l0b0]

It would be useful to have some way to show all the mutants generated by mutmut, to get an idea for what has been tried during a run. Doing this manually seems like it would be a chore:

for class in survived killed timeout suspicious skipped untested; do
    for id in $(mutmut result-ids "$class"); do
        mutmut show "$id"
    done
done

Another useful variant would be to have flags for each of the classes, such as mutmut show --survived showing all surviving mutations.