Add new option : data-filename-placement

Available values are `outside` (default) and `inside`.
When `data-filename-placement="inside"`, the selected file name will be displayed inside the button, instead of its label

```html
<input type="file" 
    title="This text will be replaced by the selected file name"
    data-filename-placement="inside" />
```

In addition, I added a `<span>` to contain button text.
I also added an example in your `demo.html` file.