add readmore tag support

In your theme replace
```
<section class="post-excerpt">
            <p>{{excerpt}}&hellip; <a class="read-more" href="{{url}}">&raquo;</a></p>
</section>
```
With:
```
<section class="post-excerpt">
            <p>{{content readmore="true"}}&hellip; <a class="read-more" href="{{url}}">&raquo;</a></p>
</section>
```

In post u can use `<!--readmore-->`

Mb it need to refactoring, but i think is good idea...