engine cat honors options echo and eval

[HanOostdijk]

I think it would be nice if the cat engine would honor the general chunk options echo and eval . The use of lang is not clear to me, but a chunk will now be echo-ed when lang is present or echo == TRUE

[yihui]

Thanks! echo = TRUE/FALSE is supported but it requires a language name to show the output, e.g.,

```{cat, class.source='js'}
var x = 1;
```

```{cat, class.source='js', echo=FALSE}
var x = 1;
```