Add post tag separator

I was looking into adding separators for multiple tags on my posts and was wondering what the thinking was behind out post tags are displayed

```html
{{#foreach tags}}
  <span itemprop="{{#if @first}}articleSection{{else}}keywords{{/if}}">{{name}}</span>
{{/foreach}}
```

In particular the individual spans for each tag and the different class names. In any event I've started just using this on 

```html
<span>{{tags separator=", "}}</span>
```

Perhaps it's just a visual preference. If you feel it doesn't add anything to ghostium feel free to reject this, I won't be hurt :)