Default all flake8 messages to warnings

Don't match on individual classifiers, to support arbitrary flake8
plugins and to remove the unreasonable classification
[Fix #545] Add an option to customize flake8 error levels
Add tags for each syntax checker

Also allow selection via `(checker FOO)' argument to test/run.el
Merge branch '545-flake8-simplification'

* 545-flake8-simplification:
  [Fix #545] Add an option to customize flake8 error levels
  Default all flake8 messages to warnings