RFR 8133948: Add 'edit' function to allow external editing of scripts

[Sundararajan Athijegannathan]

Please review http://cr.openjdk.java.net/~sundar/8133948/ for 
https://bugs.openjdk.java.net/browse/JDK-8133948

Note: ExternalEditor and EditPad classes are from kulla/jshell code - 
modified to suite nashorn jjs. Perhaps eventually code could be 
refactored and reused from jdk.interanal.le.

Thanks,
-Sundar