Add [ method for grouped_df

[hadley]

`[.grouped_df` <- function(x, i, j, drop = FALSE) {
   out <- NextMethod()
   groups <- intersect(names(out), dplyr::group_vars(x))

   if (length(groups)) {
     dplyr::grouped_df(out, groups)
   } else {
     dplyr::tbl_df(out)
   }
 }

Related to #4004