Add withTags function

This implements the DSL that we discussed with @hadley and @jjallaire.

The following are equivalent:

```
tags$div(class = "myclass",
  tags$h3("header"),
  tags$p("text here")
)


withTags(
  div(class = "myclass",
    h3("header"),
    p("text here")
  )
)
```