doc: update environment vars in manpage and --help

- Added NODE_REPL_HISTORY to the environment variables in the --help
  and made all descriptions start with lower case for consistency.

- Added NODE_REPL_HISTORY and NODE_ICU_DATA to the man page.

PR-URL: https://github.com/nodejs/node/pull/2690


Reviewed-By: fishrock123 - Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: evanlucas - Evan Lucas <evanlucas@me.com>