Add pytest_add_cli_args and pytest_add_cli_args_test_selection configs

[Otto-AA]

Closes #423, #417, #398

From my point of view, this PR is finished. But I did not commit directly, in case you have feedback.

This adds the options:

• pytest_add_cli_args: general args, e.g. changing the config file, overriding config options, disabling plugins
• pytest_add_cli_args_test_selection: test selection args, e.g. "tests/", "-k 'foo and not bar'", "-m 'not slow'", ...

Also PytestRunner.list_all_tests now only returns selected tests. If pytest_add_cli_args_test_selection excludes some tests, this would have previously also returned the excluded tests, causing trouble if mutmut is run multiple times (reported here).

I did not name it pytest_addopts because pytest_addopts expects a string, and the new configs expect a list. I am using a list, because we pass the args as a list to pytest and I did not want to re-implement the option parsing of pytest.

tests_dir is now a special case of pytest_add_cli_args_test_selection, so I've removed tests_dir from the documentation and replaced it with pytest_add_cli_args_test_selection. But tests_dir still works if it's used.