Fenced code blocks support

We’ve just added fenced code blocks support for Markdown in bug descriptions. That means you can use GitHub-style Markdown for blocks of code and get language-specific syntax highlighting. For example, to syntax highlight JavaScript code:

```javascript
function test() {
    console.log('test');
}
```

The language identifier is optional and if not provided we’ll take a guess. We’re using highlight.js to do the syntax highlighting with support for the following languages: Apache, Bash, C#, C++, CSS, CoffeeScript, Diff, HTML, XML, HTTP, Ini, JSON, Java, JavaScript, Makefile, Markdown, Nginx, Objective C, PHP, Perl, Python, Ruby, SQL.