-i --ignore flag doesn't accept a list of arguments

[Seluj78]

When I run sphinx-lint -i .venv .tox I was expecting sphinx-lint to ignore both .venv and .tox directories but it isn't the case.

For it to work, I need to do sphinx-lint -i .venv -i .tox which is kind of ugly IMO

[miketheman]

This could be done by changing from append to extend which was introduced in Python 3.8 (which is now the earliest supported version of sphinx-lint).

action="extend", nargs="+", type=str