Provide a way to disable automatic printing of results in Toree Scala

Scala REPL supports a flag, ":silent" to enable/disable automatic printing of results (those messages begin with "res"). This is also supported in Spark shell. But for Toree Scala kernel, this flag is not supported. Thus the result of a cell would be always printed.

But if there is any logging mechanism, the results would be recorded in a log file, while this may not be desirable. For instance, a user may not want any sensitive data logged when running "dataRDD.take(5)" in a cell.

Also found the possible source codes for this:
https://github.com/apache/incubator-toree/blob/master/scala-interpreter/src/main/scala/org/apache/toree/kernel/interpreter/scala/ScalaInterpreter.scala

There are three methods with silent: Boolean = false as parameter.