`selected()` removed without deprecation

1.0.0b6 renamed the `selected()` function call to [selectedContainer()](https://github.com/wymeditor/wymeditor/blob/master/src/wymeditor/editor/base.js#L500). This method is used in many 3rd-party plugins, which will be broken. Additionally, this change wasn't [documented](https://github.com/wymeditor/wymeditor/releases/tag/v1.0.0b6) in the backwards-incompatible changes.

Instead of removing it entirely, the `selected()` method should output a deprecation warning just like `html()` does indicating that it will be removed in the first 1.0.0 release candidate.