Fix broken output in non TTY mode

When using colour properties on a string when TTY mode is disabled (program.opts.useColors is false), the property value returned is the object representation of the string, instead of the string itself. See  => http://stackoverflow.com/questions/27337089/node-js-coloured-text-from-stdout-gets-converted-to-object-notation-when-writte?noredirect=1#comment43131296_27337089