Find tests based on substring matches with module/class/method names.

[qyearsley-zz]

Currently, one can specify a list of specific test modules/classes/methods to run by giving them as positional args.

Another possible behavior, which may be more convenient, would be to find all tests (all modules, test case classes and test methods) whose name includes one of the specified filter strings as a substring.

I think this would involve a change to Runner.find_tests (https://github.com/dpranke/typ/blob/master/typ/runner.py#L329)