Hookery

* Allow registering partials and functions with kwargs with defaults as handlers
* ObservableAttr example
* Check kwargs passed to trigger if hook has strict args set
* Allow passing self= to Hook methods
* No more direct calling of handlers, should call with hook context …
* No more Handler.get_result and Hook.get_result_from_handler, instead just Hook.call_handler

Allow passing `args=` to `Hook` so that when a handler is registered, its signature is checked against what `Hook` is able to provide.