Add support for Twitter

[Potherca]

To have images show up on twitter in a more desirable fashion, Twitter Cards need to be implemented. This basically means adding extra meta tags, like this:

<meta name="twitter:card" content="photo" />
<meta name="twitter:site" content="@flickr" />
<meta name="twitter:title" content="Mountain sunset" />
<meta name="twitter:image" content="http://farm8.staticflickr.com/7334/11858349453_e3f18e5881_z.jpg" />
<meta name="twitter:url" content="https://www.flickr.com/photos/reza-sina/11858349453/" />