`−` is returning `[?]`

Hi,

I'd like to report a bug, unidecode seems to fail when trying to transform `−`:
```js
var unidecode = require('unidecode');
unidecode('ABC−1')
```

Gives: ` ABC[?]1`
Expected value: ` ABC-1`

Start by running the reported require('unidecode') reproduction with `ABC−1` and compare it with the expected `ABC-1` result. Locate the transliteration entry point and mapping used for `−`; done means this input returns the expected ASCII hyphen without changing the surrounding characters.

Written by the indexing model from the issue text.