Tool does not exit when using external seeds in Java 10

[patrickfav]

If the tool is used with an external seed provider (ie. if you do not provide -o). The tool does not exit correctly but gets stuck after printing the randoms.

This could be due to the used thread pool (used with RxJava):

dice/src/main/java/at/favre/tools/dice/RndTool.java

Line 169 in 214da59

.doFinally(parallelExecutor::shutdown)

and the shutdown maybe not working.

This was observed on Win 10 with Java 10.0.1; it works on Java 1.8.0_172