Add support for description 

I implemented  #145 like this:

The data object returned by `@example` looks like
```js
{
  type : 'scss',
  description : 'description', // new key
  code : 'code'
}
``` 

Supporting this syntax 
```
@example scss - How to clamp a number.
```
and without the `-` because the `type` should always be a single word. 
```
@example scss How to clamp a number.
```