Support tying events to selectors

[aem]

Currently all events are captured at the document level which means:

1. All commands are global, there's currently no ability to make a keybinding work, say, only inside of a contenteditable element
2. Any other event higher up in the tree can stop propagation, making issues tough to debug

We should revise registerListener to allow an optional third parameter with the selector to target with the key command, while defaulting to a global handler