CLI arguments are not aligned with Pytest arguments style

[germa89]

Pytest arguments replace the spaces by minus (-) when defining arguments. For example:

pytest --durations=10 --durations-min=1.0

https://docs.pytest.org/en/6.2.x/usage.html#profiling-test-execution-duration

whereas in pytest-pyvista all the arguments are separated by "_".

This is a minor thing. But I just wanted to highlight it for consistency.

[germa89]

Mmhh... the fault handler also uses "_" so ... feel free to close if you feel like it.

https://docs.pytest.org/en/6.2.x/usage.html#fault-handler

[MatthewFlamm]

I have no strong opinion. The most important thing is to be consistent within this package and not mix the usage. Let's leave open to see if anyone has a strong opinion.