Markdown markup added

Closes #327,  ref #62 

Thanks to @rafi

At the moment following are supported:

```php
/**
 * My cool comment 
 *
 * <code>
 * Hell yeah!
 * </code>
 */
```

```php
/**
 * My cool comment 
 *
 * <pre>
 * Hell yeah!
 * </pre>
 */
```

---

**Help with ``` is appreciated**:
Preg_match mostly.
    
```php
/**
 * My cool comment 
 *
 * ```php
 * Hell yeah!
 * ```
 */
```