Allow `erb` and `haml` helpers to take a block

What do you think about this?  I have changed only `erb` and `haml` helpers because for other template engines i do not know which ones have "layouts".

This is to be able to use nested layouts using `erb` and `haml` helpers. Real nested layouts, not in the sense of Rails.

When i want to nest two layouts in my application, i currently can do like this:

```ruby
render :haml, :outer_layout, :layout => false do
  render :haml, :inner_layout1 do
    haml :page1
  end
end
```

I want to be able to do simply:

```ruby
haml :outer_layout, :layout => false do
  haml :inner_layout1 do
    haml :page1
  end
end
```