[WIP]: Add bgImageStyle prop, refactor src/util/base.js

[DDunc]

Expected functionality when complete:

• bgImageStyle allows for setting the backgroundImage property directly (resolve bgImage prop should also support gradients #428)
• Separate out the one-time dev-only warning flags/console.warns, make sure they provide useful information about what sugar props will be ignored when conflicts are present (also resolving bgImage prop should also support gradients #428).
• Tests for util/base.js (the refactor into smaller pure functions has made this easier)
• Eslint won't hate me

[kenwheeler]

Delightfully functional

[DDunc]

closing in favor of #542