Make cell-magics opt-in rather than opt-out

Currently, nbqa tries to process all cell magics except those in

https://github.com/nbQA-dev/nbQA/blob/a99e0e2413c7af340ba994f86752bb29b8b36a82/nbqa/save_source.py#L31-L51

I think we should do the opposite: never process cell magics, except for some hard-coded common ones like matplotlib inline and timeit, + add a command-line argument so users can pass extra ones they might want to process

This means deprecating nbqa_ignore_cells and instead adding nbqa_process_cells.

IMO it’s OK to make a 0.6.0 release with this breaking change in - this is a relatively new project, and it’s not like the user-base is large enough that we need to go through a deprecation cycle

cc @nbQA-dev/nbqa-core in case anyone has any comments - else I’ll try to work on during the weekend

[MarcoGorelli]

Yup - have gone with erroring in #559

[s-weigand]

IMHO making the option error or at least warn that it will be ignored, and point to this issue or somewhere in the docs is the most user-friendly and yet simple solution.