[toggle-core] Collect feature events inside write model

[kpicaza]

To allow the user to hook actions that occurred in the write model, we need to store events in the model itself to allow dispatching then later with a PSR-14 dispatcher or maybe with a custom framework dispatcher(as in Laravel's case)

Issues to close the epic:

• Add the ability to save events in the write model Feature class toggle-core#15
• Create FeatureWasCreated event and dispatch it inside Feature Class toggle-core#16
• Create AnStrategyWasAdded event and dispatch it inside Feature Class toggle-core#17
• Create AnStrategyWasRemoved event and dispatch it inside Feature Class toggle-core#18
• Create FeatureWasEnabled event and dispatch it inside Feature Class toggle-core#19
• Create FeatureWasDisabled event and dispatch it inside Feature Class toggle-core#20