styling helper that supports composite styles and caching

Summarized in the 'Styling' section in the README.md file. I'm pushing this out for review... it would need tests, and improved documentation, but I wanted to see if this is helpful to others.
Let's change `style` to be `properties`. If you want, you can allow `style` as an alias (that's how it is in ProMotion), but `properties` should be the documented version.