Add preview.rb script

preview.rb is a simple script that parses .itermcolors files and applies the colors to the current session using [iTerm's proprietary escape codes](https://iterm2.com/documentation-escape-codes.html). It runs with the default system Ruby on OSX (/usr/bin/ruby) so you don't have to install anything. But as noted in the page, it doesn't run on tmux or screen.

I find it very useful when I want to test many number of schemes

```
# Applies the schemes in turn as you press any key until you type in CTRL-C or ESC
tools/preview.rb schemes/*
```