repl: don't use tty control codes when $TERM is set to "dumb"

This change stops the REPL from using ANSI control codes for colours
when the TERM environment variable is set to "dumb".

"dumb" is the terminal type with the smallest set of capabilities as
described by terminfo. See:

http://invisible-island.net/ncurses/terminfo.ti.html#toc-_Specials

Related: https://github.com/nodejs/node-v0.x-archive/issues/5344
Related: https://github.com/nodejs/node-v0.x-archive/pull/25506


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
PR-URL: https://github.com/nodejs/node/pull/2712