Fire an unload event on removal

Plugins that need to bind events to `window` or `document` can attach a
listener for this event and unbind their event handlers.

However, for symmetry, unload is fired only if load has been fired.

See https://github.com/mapbox/Leaflet.fullscreen/commit/145938baad28aa1179c60df8da8454d3dc4235d4 for an example of use.