support a "comment/"reason" field for error mesages

[DetachHead]

i think it'd be helpful if you could add more information about banned imports in your project, for example:

[[modules]]
path = "tests.**.test_*"
cannot_depend_on = ["tests.**.test_*"]
reason = "test files should not import from other test files. consider moving the imported symbol to a separate module such as a `conftest.py` instead"

this comment would then show up with the diagnostics, like so:

[FAIL] tests\test_foo.py:54: Cannot use 'tests.test_bar'. Module 'tests.test_foo' cannot depend on 'tests.test_bar'. reason: test files should not import from other test files. consider moving the imported symbol to a separate module such as a `conftest.py` instead