Allow additional arguments

This change allows for contextfixtures to use regular fixtures or other contextfixtures in the traditional manner without needing to import them. This is a non-breaking change.

@pytest.fixture
def one(request):
    return 1

@pytest.contextfixture
def two(request):
    yield 2

@pytest.contextfixture
def main(request, one, two):
    yield one + two

def test_main(main):
    assert main == 3
    assert False