Allow unregistering Service Worker scripts

[miina]

Currently only method for registering service worker scripts exists, however, it should also be possible to unregister scripts.

As on example this could be used if a plugin wants to create their own script for a feature (such as offline commenting) as opposed to using the default -- having more than one matching routes would show a warning in console and only use the first match omitting the others.

[westonruter]

Yes.

The other option would be to override the src for an existing registered script, but unregistering should also be possible. For the replacement, the same script handle should probably be used in case of other plugins registering service worker scripts which have it as a dependency.

This is related to #33 in that perhaps there should be a capability to have a service worker script registered but not enqueued. In this way there could be service worker functionality that is defined and available in core, but which isn't necessarily used.