Add --extend-exclude parameter

[thejcannon]

Adding the --extend-exclude parameter to add additional exclude regexes on top of the --exclude regexes. This is especially useful if you want to specify additional files/directories on top of the very useful default ones (as demonstrated by the change to our own pyproject.toml).

The parameter name also matches flake8's parameter of --extend-exclude.

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Fixes #1571

[ichard26]

To be clear, I want to do a review and already have a comment in mind, but I unfortunately don't have time to put it in words right now, so I would appreciate holding off on the merge for the time being. Thanks!

[ichard26]

Approving the changes generally, but there are a few nits and minor issues that would be nice to have resolved :)

I was going to complain about how someone can pass both --exclude and --extend-exclude at the same and how it would be confusing, buuuutttt... after some more thinking, there's some valid use cases for this configuration design.

Thanks for submitting an excellent PR! Thank you for the time investment, it really helps out the project. Why not add yourself to the AUTHORS list at the end of the README?

[JelleZijlstra]

@ichard26 are you OK with merging this now? We can handle the click callbacks for regexes in a separate PR (as you said above). Thanks for your work @thejcannon!

[ichard26]

@JelleZijlstra LGTM! Thanks for checking in with me, you didn't have to. Thanks @thejcannon for the good work!

Look ma! I contribute to open source!

p.s. thanks for the little fun you portrayed in this commit message, it's a little light in what's a very dark day for me :)