[Improvement] Add HTML attributes to containers

Hello,

This is not an issue but an improvement. It would be nice to add HTML attributes to `containers` to layout files.

For example, if I want to create the following HTML output with a container:

```
<header class="header" id="header" data-sticky data-isheader="true">

</header>
```

I can specify tag, id and class but not the other attributes.

What do you think about it?