Add `--exclude-migrations-from`

[restaste]

Hello, and thanks for that great tool!

We're in the process of introducing migrations linting in our CI pipelines (to move towards only bw-compatible migrations, to further eliminate downtime).

We don't want to commit to 100% bw-compatible migrations just yet, so we'll need to rely on ignoring migrations. All of the currently available methods pose a small problem for us:

• linter.IgnoreMigration() means our migration scripts need a reference to the linter. We'd prefer to keep things separate (i.e. have migrations contain only migration-related concerns).
• --ignore-name means our CI scripts will start containing migration file names and will quickly grow as we add non-bw-migrations.
• --ignore-name-contains means we can't name our files however we want.
• --include-*: we want to default to "all migrations are linted" and specify exclusions individually.

Notably, there's --include-migrations-from which I feel is exactly what we'd want, but for exclusion: putting migration file names in a dedicated file clearly separates the concerns.

So I wonder whether you considered adding an --exclude-migrations-from option, which would take a file containing names of migrations to exclude from linting? I guess one open question would be how this plays with the other include/exclude options.

Arguably, none of our small gripes above are hard blockers (we'd probably start with --ignore-name), but you're open to it, I'd happily set some time aside to work on this.

Also: maybe I missed something and my concerns aren't justified. Feel free to educate me if that's the case!

[restaste]

I spent some more time reading through the docs and ended up putting all my exclusions in a .django_migration_linter.cfg file, which worked out exactly how I wanted.