Allow for opt-out of the java shadowRuntimeElements variant

[m8nmueller]

Add a extension property to regulate whether a new variant is added to the java component. I must admit I don't know exactly why the variant was added. The commit message was only about support for Gradle 5 -> 6. So this is just a proposal.
Closes #651

[m8nmueller]

Hi @johnrengelman,
I was trying to look after another "problem" if you can call it one: The shadowRuntimeElements configuration lists the shadow jar artifact twice (see native outgoingVariants task) - one inherited from the shadow configuration, the other one being the output. Looking at the code, I wondered why shadowRuntimeElements is needed at all. Wouldn't it be possible to just reconfigure the shadow configuration accordingly?