Request: Add app.logger support

[ngandhy]

Pylint gives "method logger has no "warning" member" when trying to use app.logger.warning("test")

https://stackoverflow.com/questions/51678585/pylint-false-positive-for-flasks-app-logger-e1101-method-logger-has-no-d

[fmigneault]

Relates to pylint-dev/pylint#2586