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.

With TTY enabled:
![](http://i.stack.imgur.com/qiiLx.png)

With TTY disabled:
![](http://i.stack.imgur.com/sKFS9.png)

See  => http://stackoverflow.com/questions/27337089/node-js-coloured-text-from-stdout-gets-converted-to-object-notation-when-writte