added custom doc and win functionality

This helps to load the editor in a parent frame but edit inside an iframe.

This has been discussed before here #183 and here #269. The scenarios where this can come handy is where you are developing doing a drag and drop editor where the text is edited inline but everything happens inside an iframe, completely isolated from the upper frame.

You still need to override the `setToolbarPosition` in your side of things, a future improvement could be pass functions to calculate this:

```javascript
this.toolbar.style.top = buttonHeight + boundary.bottom - this.options.diffTop + offset - this.toolbar.offsetHeight + 'px';
```

But that's another story.