RFR 8148316: jshell tool: Configurable output format, et. al. - v9

[Robert Field]

Update:
   Per Brian's review have added setting prompt -- /set prompt
   Per Jan's review have nice consistent verbose /help support and error 
messages, which meant rolling in the /set unification,
   have moved feedback support to its own class, added much more 
extensive testing including invalid input, and addressed the other 
smaller issues.
   Detailed help is available, for example:  /help set format
   To achieve clean errors, made new feedback mode creation explicit -- 
/set newmode

Bugs (the second (unrelated) bug was discovered and fixed in the process 
of testing, the third bug was added to address output consistency)
     https://bugs.openjdk.java.net/browse/JDK-8148316
     https://bugs.openjdk.java.net/browse/JDK-8149524
     https://bugs.openjdk.java.net/browse/JDK-8148317

Webrev:
     http://cr.openjdk.java.net/~rfield/8148316v9.webrev/

The example output configuration has been updated accordingly (Open with 
/open):
     http://cr.openjdk.java.net/~rfield/formats/arrowResultNlMeta.jsh

Thanks,
Robert