Add support for multiple delimiters

If you want to create a "tag selection" component you might expect a behavior where string splits not only at ```,``` but on other characters like ```Space```.

This PR adds the functionality to define multiple delimiters as an array. All given delimiters are considered when splitting the string. The first delimiter is used to concatenate values.

```javascript
// Usage with one delimiter (100% compatible)
$("#elem").selectize({
    delimiter: ','
});

// Usage with multiple delimiters
$("#elem").selectize({
    delimiter: [',', ' ']
});
```