Added support for colorized values

When I wanted to colorize some values, I used the following formatter

```ruby
  class ColorFormatter
      def initialize(color)
        @color = color
      end
      def format(data)
        HighLine.color(data.to_s, @color)
      end
    end
```

The cells were then padded incorrectly, because the unprintable escape sequence that sets the color was counted in the length of the value. this patch is stripping the unprintable stuff during padding.