Require --app or --slice name when app has at least one slice

Recently, I forgot to include `--slice name` so it generated the files in `app/` when I was expecting them to be in a slice.

Before users have any slices, we can let them run it without either option though, since we know they want it in `app/`.

Once #185 is implemented, we can also suggest running generators from within the directories in lieu of providing these options