Optionally accept `locals`  in handler function

i.e. instead of 

```R
app$use(function(req, res) {
  locals <- req$app$locals
  if (is.null(locals$num)) locals$num <- 0L
  locals$num <- locals$num + 1L
  "next"
})
```

It would be nice to write

```R
app$use(function(req, res, locals) {
  if (is.null(locals$num)) locals$num <- 0L
  locals$num <- locals$num + 1L
  "next"
})
```