Fine-grained rule exclusions

[TheMrMilchmann]

In some cases, it is desirable to disable specific rules for certain classes. When using ArchUnit regularly, this would typically be achieved by filtering the JavaClasses before checking the rule.

It would be nice if the plugin would expose a DSL to replicate such exceptions on (at least) a per-rule basis. Ideally, this would tie into a general mechanism for per-sourceset configuration (See also #90).

While covering the entire expressiveness of ArchUnit in a DSL is probably not feasible, it would be nice a nice start if we could ignore targets by name or when they have certain annotations.

[wakingrufus]

Thanks for the feedback! I doubt it would be feasible to filter based on the source file content, but based on path/name should be doable. I have had some issue in the past with the gradle filter API and configuration cache (in the ktlint-gradle project), so I want to be careful about introducing this, and make sure we do it the right way.

[TheMrMilchmann]

I spiked a small change of what I had in mind in #99. Let me know what you think, and I'll flesh it out if we agree on the approach.