Add the ability to skip checking some functions.

[mgilson]

There are times when you might not want to check a function's
docstring. Specifically, we frequently want to ignore
test files. There is already an exemption that causes test
functions to be considered non-public. This extends on
that idea and makes it configurable so a user can cause
a test function to be allowed to have no docstring.

This is a potential implementation for #537, #515

Thanks for submitting a PR!

Please make sure to check for the following items:

• Add unit tests and integration tests where applicable.
  If you've added an error code or changed an error code behavior,
  you should probably add or change a test case file under tests/test_cases/ and add
  it to the list under tests/test_definitions.py.
  If you've added or changed a command line option,
  you should probably add or change a test in tests/test_integration.py.
• Add a line to the release notes (docs/release_notes.rst) under "Current Development Version".
  Make sure to include the PR number after you open and get one.

Please don't get discouraged as it may take a while to get a review.

[aphedges]

From looking at the code itself, this looks good except for a small concern. I'll try it out later this week to see if it works well.

[aphedges]

search() is probably a better option than findall() because it only needs to find one instance of the pattern instead of all. However, function names should short enough that the performance difference is negligible, so I'm not sure a change is needed.

[mgilson]

Yeah, I was just following along with the prior art for decorators (e.g. line 151).

I'm happy to change just this one (or both) to use pattern.search instead.