Be able to change default className `rxv-container` applied to root element

The className `rxv-container` is applied to the root element. However that might conflict with an existing className in someone else's codebase. 

Allow the user to change that className to something else by passing a prop. For example:

```
<XMLViewer xml={xml} defaultClassName="not-rxv-container" />
```

DOM output:

```
<div className="not-rxv-container">
   ...
</div>
```