repl: persist history in plain text

Persists the REPL history in plain text using the new NODE_REPL_HISTORY
environment variable. Deprecates NODE_REPL_HISTORY_FILE. The REPL will
notify the user and automatically convert the history to the new format
if files are specified.

PR-URL: https://github.com/nodejs/io.js/pull/2224


Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>