[Minor Fix] Ensure we suppress events for all keys we handle

The logic in [content_scripts/vimium_frontend.coffee](https://github.com/philc/vimium/blob/master/content_scripts/vimium_frontend.coffee) didn't match up with the [background_scripts/main.coffee](https://github.com/philc/vimium/blob/master/background_scripts/main.coffee) implementation; we were handling keys but not suppressing the events if the user started typing another command midway through typing a multi-key shortcut.