Support un-'px'ed styles

We should support the same CSS style objects as React. For example, we should allow for `{ width: 10}` to be a valid style object.

In general, why don't we utilize `CSSPropertyOperations.createMarkupForStyles` to generate a string of CSS? We could call it after checking for pseudo-selectors and media queries, and performing whatever pre-processing we want to support. I've already implemented this and can merge it in, if you agree.

The issue names no files or tests; start by locating the style-object serialization entry point and compare its handling with React's CSSPropertyOperations.createMarkupForStyles. Done means numeric values such as { width: 10 } serialize as valid CSS while pseudo-selectors, media queries, and existing preprocessing continue to work.

Written by the indexing model from the issue text.