Right-to-left format method

R seems to have some support for right-to-left scripts, presumably picking up on the unicode bidi class, e.g.:

```r
library("numerals")
numeral(1:9, "ar")
#>
#> [1] ١ ٢ ٣ ٤ ٥ ٦ ٧ ٨ ٩
```
But it doesn't always happen when expected:

```r
numeral(1:9, "fa")
#>
#> [1] ۱ ۲ ۳ ۴ ۵ ۶ ۷ ۸ ۹
```

And it would also be nice if the indices and justification followed, something like this:

```r
library("numerals")
numeral(1:9, "ar")
#>
#> ١] ١ ٢ ٣ ٤ ٥ ٦ ٧ ٨ ٩]
```