Scope findMatchingSelectionParent to current editor

When looking for parent elements, do not traverse past the nearest containing medium editor.

For context, I'm using medium-editor with nested editable elements as in the screenshot below. Prior to this change, selecting text within the nested editor would not trigger the toolbar because the editor was a child of an element with editing disabled.

![542ac054c38aa5631ff562cd](https://cloud.githubusercontent.com/assets/438574/4460019/8bee78de-48af-11e4-81ef-96e19fcc0aef.png)