Separate render and style params from building

[rnystrom]

Love this blog post! Removing all color/font from building would be amazing. Instead:

• Use "style builder" pattern that registers styles
  • Need to still be able to "push" and "pop" styles
  • Should all props be in one style (.font, .foregroundColor, etc), or should there be a style per prop? Latter seems harder to maintain
• Remove UIContentSizeCategory from all params until render

Necessary to make progress towards GitHawkApp/GitHawk#198 so you can have themes/styles and re-render (missing all caches) w/out having to rebuild everything (which you'd have to do today).