Implement multi-regional and multilingual URLs

To help search engines understand the language or regional of URLs, and build up the equivalence relationships between different languages or regional sites. 

Based on Google's suggestion, I add the support of alternate tags like below:

```html
<link rel="alternate" hreflang="en" href="http://example.com/" />
<link rel="alternate" hreflang="de" href="http://example.com/de/" />
```

This should close #40.