Implement `pick()`

[markfairbanks]

From dev dplyr.

library(dplyr, w = FALSE)

df <- tibble(x = 1:3, y = 1:3, z = c("a", "a", "b"))

df %>%
  mutate(row_sum = rowSums(pick(x, y)))
#> # A tibble: 3 × 4
#>       x     y z     row_sum
#>   <int> <int> <chr>   <dbl>
#> 1     1     1 a           2
#> 2     2     2 a           4
#> 3     3     3 b           6