Please make it possible to override the rerun count globally

Consider the following sample: ```python import pytest @pytest.mark.flaky def test_foo(): assert False @pytest.mark.flaky(reruns=3) def test_bar(): assert False ``` This causes the tests to be rerun exactly once and 3 times, independently of `--reruns=...` parameter on the CLI. I would find it really helpful to be able to override the upstream defaults somehow via the command-line, as our downstream workflows often suffer from much more flakiness than the pristine upstream CI.