[Feature Request] Add the ability to destruct the editor

If there is a place where we are conditionally showing the editor, then it is better to have a destruct method.

```javascript
var simplemde = new SimpleMDE({ element: document.getElementById("MyID") });
```

And when there is a need to hide/remove the editor:
```javascript
simplemde.destruct();
```

It should remove all the elements that were added because of the construct method.