Gradle plugin: ability to set the sourceset to which generated files are added

[Fabilin]

The jsonschema2pojo gradle plugin automatically attaches the generated files to the main sourceset of a project. However, some projects may wish to add generated sources to a separate sourceset, or even to generate them without using them as sources at all. As such, it would be helpful to add a (nullable) sourceSet variable to the plugin's configuration, which could default to the main sourceset.