This is so that I can run `tox -- -n 8` to run the tests in parallel, or
`tox -- tests/checker/test_misc.py::TestMisc::test_html5` to run just a
single test, without having to repeat all the other options.
I haven't moved --cov=linkcheck because I don't want coverage results
when I'm limiting the test run to a single test (they just make the
interesting bit -- the test result itself -- scroll up).
I've also added -ra to the default option list because then several
tests fail, I'd like to see a list of their names in one place, not
spead out between the huge tracebacks.
