Use type provider for nodes with identifiers

We could make nodes with IDs accessible as follows:

``` scala
val root = html("test.html")
val div = root.ids.text  // instead of: tpl.byId[tag.div]("text")
```

[Type providers](http://docs.scala-lang.org/overviews/macros/typeproviders.html) are an option, but are not well-supported by IDEs.
