Add `add_javascript` and `add_stylesheet` on TurnipFormatter and TurnipFormatter::Template

Usage

```
TurnipFormatter.configure do |config|
  config.add_stylesheet File.dirname(__FILE__) + '/foo.css'
  config.add_javascript File.dirname(__FILE__) + '/bar.js'

  config.add_stylesheet 'http://example.com/baz.css'
  config.add_stylesheet '//example.com/piyo.css'
end
```
CHange RSpec project_name to Turnip Configuration title.
Add configuration on README.