Feature request: Enhance plugin options to only consider certain source folders

[writtmeyer]

With the Gradle plugin you can configure which source folders to use for analysis. We need something like this to be available in the plugin as well, otherwise we might see plenty of problems in the IDE that are not really relevant for the project.

Slightly related to this bug/feature request: #14. But since it's only a subset and a separate folder selection option in the plugin would suffice, a new feature request IMHO is okay.