Yielding from a partial

With this change one can say (this is in Slim):

```ruby
  == partial :_my_partial do
    a href="/mylink" This is a link
```

where _my_partial.slim could be

```ruby
  div class="container"
    == yield
```

tesulting in
```html
  <div class="container">
    <a href="/mylink">This is is a link</a>
  </div>
```

Thus allowing caller to use the partial in layout-like manner.