push to the internal buffer?

... currently possible?

If not, the change I appended exposes the internal buffer (in a way how [jade-ext](https://github.com/1602/jade-ext) did it). 

Use Cases:

e.g.

```javascript
- formFor(user, function() {
  p
    - this.label('name')
    - this.textField('name')
- })
```

... or as discussed at #136