Accept CSS versions of style property names instead of JavaScript versions

[75th]

Animations would be much friendlier if they allowed background-image instead of requiring backgroundImage, etc. But I don't fancy writing that code, and I would really like to keep the number of dependencies to zero. Don't know how to square that circle.

[75th]

If I'm going to make this work with CSS variables (#16), I might as well just mandate the CSS versions of property names. Will make my job a lot simpler.