Append a/an to word

## Details

This function uses crude logic to append 'a' or 'an' to numbers and phrases.

words that start with aeiou

negative numbers always start with 'a', ex: 'a -3' or 'a -8'

decimals always start with 'a' ex: 0.4 is usually pronounced 'a zero point four' or 'a point four'

numbers starting with 8 are always 'an'

if the integer that comes after thousand or million is 11 or 18 then 'an'

18,000 becomes 18 and that becomes 'an 18'

if the integer that comes after thousand or million is in 1, 10, 12, 13, 14, 15, 16, 17, 19 then 'a'

15,500 becomes 15 and that becomes 'a 15'

otherwise 'a'

## Examples

```
add_article("increase")
#> [1] "an increase"
add_article("decrease")
#> [1] "a decrease"
add_article(c(1, 8, 10, 11, 18, 20, 80))
#> [1] "a 1" "an 8" "a 10" "an 11" "an 18" "a 20" "an 80"
add_article(18123)
#> [1] "an 18123"
stats::setNames(
add_article(1.8 * 10^(1:7)),
prettyNum(1.8 * 10^(1:7), big.mark = ",")
)
#> 18 180 1,800 18,000 180,000 1,800,000
#> "an 18" "a 180" "a 1800" "an 18000" "a 180000" "a 1800000"
#> 1.8e+07
#> "an 1.8e+07"
```