add opts.open and opts.close

This PR will allow you be specify an optional opening tag and closing tag. This will allow you to do stuff like this

``` js
var handlebars = require('handlebars-stream')

var stream = handlebars('<li>{{hello}}</li>', {open:'<ul>', close:'</ul>'})
stream.write({hello:'world'})
stream.write({hello:'mundo'})
stream.end()
stream.pipe(process.stdout)
```

Which will produce something like

``` html
<ul><li>world</li><li>mundo</li></ul>
```