Towel Hook

Implemented from scratch, following the general concept of the [hook by SergeyKarpushin](https://www.thingiverse.com/thing:2365512).

This plate-and-hook setup makes the sliding mechanism much larger (and thus less fiddly to print).

My printer sometimes prints with a slight elephant's foot, so this also reduces the impact of that a little (for the plate more than the hook - tighten up the lower corners of the hook with a dremel if it doesn't slide in easily).