Changing of behaviour for attributeChanged

[treshugart]

The current behaviour is to not fire for existing attributes. See: https://github.com/skatejs/skatejs/blob/master/README.md#attribute-1. However, they've recently decided on some changes.

However, there have been several issues raised that are currently open that we should keep track of and action once they've been closed:

• attributeChanged callback should be invoked on every attribute upon creation WICG/webcomponents#364 and Changes to element creation during parsing per F2F WICG/webcomponents#366
• Add attributeFilter WICG/webcomponents#367

If this list grows, we should split these out but until it's final this may serve as a better aggregation point.

This is also dependent upon #511.