Use external css file

```ruby
TurnipFormatter::Template.add_css_file('http://example.com/turnip.min.css')
#
# => '<link rel="stylesheet" href="http://example.com/turnip.min.css">'
#

TurnipFormatter::Template.add_css_file('./turnip.min.css')
#
# => ' body { font-size: 12px; }'
#
```