Common initialisms in kace.Snake

Hi,

Thanks for sharing this package. Do you think it is possible to support common initialisms in `kace.Snake` as well? For example:

``` go
fmt.Println(kace.Pascal("http url"), kace.Snake(kace.Pascal("http url")))
// HTTPURL httpurl
```

The expected output is `HTTPURL http_url`. This is needed for generating SQL column names from Go field names.