Turn Colors on and off, select wether to use -e

As you noted in https://github.com/visionmedia/n/issues/12#comment_932862: "maybe we just need some kind of option or env var to omit colors". Here is the way to do just that. One sets the output mode in the config file (which may be moved away from the bin directory) by changing the `$colors` variable to either of `"colors"` (normal color output), `"colors+"` (color output with -e) or `"nocolors"` (no colors shown). I hope you don't find to many flaws.

**Sidenotes:**

- Setting `$BLACK = $RED = $GREEN = $SPECIAL = ""` in the nocolors case is not mandatory. But I realized this too late so I left it in.
- The -e setting may have been done more elegantly with an alias. But again... I realized it this mornig... yesterday must have been late.