Add scope to bannedDependencies rule

It would be fine if this enforcer rule could be tuned more finely to fail only if some dependency is put with some specified scope (the same could be done for the optional tag, btw).

In fact, in my case, I'd like to be sure that no project of ours is using log4j as a compile dependency, but it is allowed as a runtime one.
Another example: no project should ever have servlet-api as a dependency, with another scope than provided.

Thanks in advance.
Cheers.