[SPARK-15697][REPL] Unblock some of the useful repl commands.

## What changes were proposed in this pull request?

Unblock some of the useful repl commands. like, "implicits", "javap", "power", "type", "kind". As they are useful and fully functional and part of scala/scala project, I see no harm in having them either.

Verbatim paste form JIRA description.
"implicits", "javap", "power", "type", "kind" commands in repl are blocked. However, they work fine in all cases I have tried. It is clear we don't support them as they are part of the scala/scala repl project. What is the harm in unblocking them, given they are useful ?
In previous versions of spark we disabled these commands because it was difficult to support them without customization and the associated maintenance. Since the code base of scala repl was actually ported and maintained under spark source. Now that is not the situation and one can benefit from these commands in Spark REPL as much as in scala repl.

## How was this patch tested?
Existing tests and manual, by trying out all of the above commands.

P.S. Symantics of reset are to be discussed in a separate issue.

Author: Prashant Sharma <prashsh1@in.ibm.com>

Closes #13437 from ScrapCodes/SPARK-15697/repl-unblock-commands.

repl/scala-2.11/src/main/scala/org/apache/spark/repl/SparkILoop.scala

@@ -70,7 +70,7 @@ class SparkILoop(in0: Option[BufferedReader], out: JPrintWriter)
     echo("Type :help for more information.")
   }
 
-  private val blockedCommands = Set("implicits", "javap", "power", "type", "kind", "reset")
+  private val blockedCommands = Set[String]("reset")
 
   /** Standard commands */
   lazy val sparkStandardCommands: List[SparkILoop.this.LoopCommand] =