v0.16.0

New features:
- custom notification levels can be added
  commit 14467559b9ae283cbd95cd194fbfcad54243c941
- UiNotifier can be used without ConfigFile
  commit 33dea31bfb7c95dbedcd2b7c9b8b1a97242da36f
- added commands and ignore_commands paremeters to ConfigFile.save
  commit eb1544e9648d14613cde136cea14e6c1ec9fa034
- added ignore_commands parameter to ConfigFile constructor
  commit d0ab7bf3d5c79149ce38eeea9b4b70e7040bc92d
- abstract classes can be passed to commands parameter of ConfigFile constructor
  commit 079c37c1bd486133aefc76728b2958ea5dd7b7c2

Changes:
- it is no longer possible to iterate over NotificationLevel instances with `for lvl in NotificationLevel`
  use `for lvl in NotificationLevel.get_instances()` instead
  commit 14467559b9ae283cbd95cd194fbfcad54243c941