* Improve the `doodad convert` command to convert a series of input
images into multiple Frames of a Doodad:
`doodad convert frame1.png frame2.png frameN.png output.doodad`
* Add the initial round of dev-asset sprites for the default Doodads:
* Button, Button-TypeB and Sticky Button
* Red, Blue, Green and Yellow Locked Doors and Keys
* Electric Door
* Trapdoor Down
* Add dev-assets/palette.json that defines our default doodad color
palette. Eventually the JSON will be used by the `doodad` tool to give
the layers meaningful names.
