Fix .id arg missing in imap_dfr

[cderv]

This is a small PR that fixes #429

• .id argument in imap_dfr is now took into account (see reprex below)

• unused .id argument in imap_dfc is now suppressed

• Verifying if it was all, I found that in flatten_df* verbs there was no check if dplyr is installed or not as in map_df*, map2_df*, and pmap_df*. As it is small changes, I have included in this PR rather than opening a new one

Checking the fix

library(purrr)
as_tibble_custom <- function(value, name = NULL) {
  tab <- tibble::as_tibble(value)
  if (! is.null(name)) {
    tab$col_id = paste0("col_", name)
  }
  tab
}
# with imap it does not work
list(a = 1, b = 2, c = 3) %>%
  imap_dfr(as_tibble_custom)
#> # A tibble: 3 x 2
#>   value col_id
#>   <dbl> <chr> 
#> 1  1.00 col_a 
#> 2  2.00 col_b 
#> 3  3.00 col_c
list(a = 1, b = 2, c = 3) %>%
  imap_dfr(as_tibble_custom, .id = "name")
#> # A tibble: 3 x 3
#>   name  value col_id
#>   <chr> <dbl> <chr> 
#> 1 a      1.00 col_a 
#> 2 b      2.00 col_b 
#> 3 c      3.00 col_c

[hadley]

House style is to not put package names in back ticks, and to put () after function names. Can you please change?

[cderv]

Oh sorry for that. I will definitely remember that for next time.
And revise the tidyverse style guide.
Correction is done.