Allow users to choose warnings only; misc

* Enable warnings-only filtering
* Change option names e.g. displayer -> output
* Update README - add new use case, and info on interface changes
* Add warning status and title to output of advice list
* Misc renaming inside code e.g. message_level -> detail_level
* Add misc additional internal code docs
* Bump version
2 jobs for master
Status Job ID Name Coverage
  Build
canceled #870
build_a

 
  Test
canceled #871
test_a