Allow accessing phoenix via (global) key

[point-source]

In the case that a developer is building an app with redux state management, he/she may want to call phoenix from an action which does not have clear access to widget context. In this case, it may be useful to access it via global key.

[themightychris]

I'm trying to call rebirth() from a message handler under flutter_driver, and similarly don't have access to a build context there

[MrLepage]

Will add a key parameter in the next version.