repl: default persistence to ~/.node_repl_history

Makes the REPL persistently save history by default to
~/.node_repl_history when in terminal mode.
This can be disabled by setting NODE_REPL_HISTORY="".

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>