RFR 8157200: jshell tool: Add /retain command to persist settings

[Robert Field]

Please review...

Bugs:
         8157200: jshell tool: Add /retain command to persist settings
         https://bugs.openjdk.java.net/browse/JDK-8157200

         8156910: jshell tool: crash when code with syntax error 
contains format specifier
         https://bugs.openjdk.java.net/browse/JDK-8156910

Webrev:
         http://cr.openjdk.java.net/~rfield/8157200v0.webrev/

Thanks,
Robert