Allow to parametrize IdentifiersShouldNotMatchKeywords rule with exceptions

In our _legacy_ codebase we use some common terms like "Readonly", in number of locations, and this is common terminology used by our project and our customers.

While we would like to enable IdentifiersShouldNotMatchKeywords rule, we would like to add couple exceptions in our .editorconfig, not to poison the whole codebase with `#pragma warning disable CA1716` followed by `#pragma warning enable CA1716`

### Describe the solution you'd like
Respect some property in .editorconfig with comma-separated list of keywords to bypass validation. Maybe have an optional extra list to add to validation.