Handmade hero already has a very limited skeletal animation system -- the Hero is built from a body, cape and head that are separate bitmaps drawn together with their own offsets. You could try, say, animating the head offset to make it bob up and down as the hero walks around. I suspect that since the hero has no arms or legs, Casey is probably going to try to keep the animation system pretty simple.
If you want to learn about this, I'd suggest trying a tool such as "Spriter" or "Spine" which support 2d skeletal animation rigs. Try to write something that can load and playback the example files from one of those, first using their sample importers, and then try writing it again from scratch.