Verify that the site.url has a protocol in feed.xml

Following the discussion on #66.  
I know you said that it's not really an issue. And I agree. But since your theme might still be used on a lot of github pages and that github offers https, I thought I might have a look at it and come with the solution you suggested.  
So, if there is no protocol mentioned at the beginning of  ```site.url```, it will add ```http:``` as default. Else, it'll do nothing.