Slugified emoji turns out as an empty string

Hi, thanks for this awesome library!

We've faced a situation with strings only made of emojis.

**Actual behavior**
```es6
limax('😵️') === ''
```

**Expected behavior**
```es6
// Replace by emoji name
limax('😵️') === 'dizzy_face'

// Or by emoji code (as when pasted as an URL in the browser)
limax('😵️') === 'xn--w38h'
```