Allow beforeDeactivate to return promise, and create similar lifecycle method for before activation

[robframpton]

Currently the beforeDeactivate lifecycle method of Screen can only return a boolean. It should be able to return a promise as well.

In addition to this improvement, we should add a beforeActivation lifecycle method to Screen. This will behave very similarly, but simply before that screen is navigated to.

The argument for these additions can be found at deprecate/metal-router#15.