Add mix dialyzer flag --list-unused-filters

This option will result in an error status code, so circleCI will not pass if dialyzer encounters any errors.

Author: EasterPeanut

.circleci/config.yml

@@ -25,6 +25,7 @@ jobs:
             - v1-plt-cache-{{ checksum "mix.lock" }}
             - v1-plt-cache
       - run: mix dialyzer --plt
+      - run: mix dialyzer --list-unused-filters
       - save_cache:
           key: v1-plt-cache-{{ checksum "mix.lock" }}
           paths: