stri_*_first/last_charclass - merge arg

Now it works like this:

``` R
stri_extract_last_charclass("abc123","\\p{Nd}")
[1] "3"
```

and we could add merge parameter (like in extract_all function) so it could work like this:

``` R
stri_extract_last_charclass("abc123","\\p{Nd}", merge=TRUE)
[1] "123"
```