Support knitr blocks

[hadley]

e.g.

@details

```{r}
1 + 1
```

Would yield

\preformatted{
1 + 1 
#> [2]
}

For now, code will have to run quickly as results would not be cached.

[gaborcsardi]

Interesting idea. Which environment would they run in?

[hadley]

New environment with package loaded?

[tbates]

Would certainly be good for catching errors in code given outside the examples section