Provide option to remove "-javaagent" argument passing to retrolambda

[bumfo]

retrolambda has problem running on Java 9/10 when -javaagent argument is provided.

however, it seems that there's no interface to prevent gradle-retrolambda from sending such argument.

[evant]

It shouldn't be passing -javaagent if you are using retrolamda 1.6.0+. What's your setup?