Make button text configurable; change placeholder

This uses "emptyText" and "buttonText" so you would do something like this:

```html
<input type="file" data-emptytext="No file uploaded...yet!" data-buttontext="Pick a file, idiot." />
```

Also has different default button text depending on whether it's "multiple" or not. ("Choose File" vs "Choose Files")

/cc @aebsr