This is a very simple attempt at providing the option to hide unoccupied
tags in the `river/tags` module.
It adds the option `hide-unoccupied` to hide the buttons when a tag is
unoccupied.
ISSUES: This change currently has an issue due to the way that waybar
initialises the bar using the `showall()` function. This causes all the
tags to be visible by default until an update operation happens.
