* update README with new info
* setup.sh: update arg parsing to add support for `flutter-version-file` as `-f pubspec.yaml`
* action.yaml: add flutter-version-file
* setup.sh: require `yq` only when -f option is specified
* fix tests and add a new one
* enable shell options: `errexit` and `nounset`
* move falling back to defaults to single place - setup.sh
* action.yaml: reduce number of arguments in the last invocation
