Add snip_list(na_rm) argument

[yjunechoe]

This PR adds an option to snip_list() to control whether NA values are counted as an item in the list. This is implemented in pb_str_catalog() which receives na_rm passed down from snip_list().

# "high" coded as NA, ignored in report with `na_rm = TRUE`
small_table %>% 
  select(f) %>% 
  mutate(f = ifelse(f == "high", NA, f)) %>% 
  create_informant() %>% 
  info_columns(
    columns = f,
    `Items` = "This column contains {values_f}."
  ) %>%
  info_snippet(
    snippet_name = "values_f",
    fn = snip_list(column = "f", na_rm = TRUE)
  ) %>% 
  incorporate()
#> 
#> ── Incorporation Started - there is a single snippet to process ───
#> ✔ Information gathered.
#> ✔ Snippets processed.
#> ✔ Information built.
#> 
#> ── Incorporation Completed ────────────────────────────────────────

[rich-iannone]

LGTM!