Not able to set the default placeholder as an empty string

Passing `placeholderChar=''` does not work.
For example, if you do:
```js

```
the placeholder will still be `_` instead of an empty string, while [inputmask-core](https://github.com/insin/inputmask-core) supports empty string it according to [this line](https://github.com/insin/inputmask-core/blob/master/lib/index.js#L204)