Auto-completion in roxygen comment

It would be nice to have auto-completion of roxygen documentation entries such as `@params` and `@export`.

```r
#' Test function
#' @param x an object
#' @param check a function
#' @export
#' @examples
#' test(NULL, is.null)
#' test(0, is.null)
test <- function(x, check) {
  if (check(x)) {
    x
  } else {
    stop("Check failed")
  }
}
```
