Add options to stratego-runtime configuration

[Virtlink]

This PR adds the following options to the stratego-runtime CFG configuration block:

• class-kind to specify the class kind (Generated or Manual)
• with-primitive-library to specify one or more implementation of IOperatorRegistry (Stratego primitive library) to be loaded
• base-StrategoRuntimeBuilderFactory to override the package and name of the (generated) StrategoRuntimeBuilderFactory implementation
• extend-StrategoRuntimeBuilderFactory to specif the package and name of the (manual) StrategoRuntimeBuilderFactory implementation

This PR also adds the documentation.

Usage example for with-primitive-library:

stratego-runtime {
    with-primitive-library = java mb.tiger.MyPrimitiveLibrary
}

package mb.tiger;

import org.spoofax.interpreter.library.AbstractStrategoOperatorRegistry;
import javax.inject.Inject;

public final class MyPrimitiveLibrary extends AbstractStrategoOperatorRegistry {
    @Inject public MyPrimitiveLibrary() {
        add(new MyPrimitive());
    }
    @Override public String getOperatorRegistryName() {
        return getClass().getSimpleName();
    }
}

Usage example for overriding the generated classes:

stratego-runtime {
    class-kind = Generated
    base-StrategoRuntimeBuilderFactory = java mb.tiger.MyTigerStrategoRuntimeBuilderFactory
    extend-StrategoRuntimeBuilderFactory = java mb.tiger.MyExtendedTigerStrategoRuntimeBuilderFactory
}