Document not defined

Font-picker is used as a dependency in react-font-picker. Building the project fails due to document not being defined at build time. You need to wrap lines 158-159 of `index.es.js`  as
```
if(typeof document !== 'undefined'){
    var previewFontsStylesheet = document.createElement("style");
    document.head.appendChild(previewFontsStylesheet);
}
```