Code panics for Unicode U+10000

This code panics:

```go
slug.Make("𐀀")
```

This is due to the internal use of `rainycape/unidecode`, which can not handle Unicode character U+10000, see https://github.com/rainycape/unidecode/issues/8.

Given the inactivity of the upstream package, it might be worth filtering out the character in question before passing it to `unidecode`.