Quote the value of the content property

Only automatically quote the value of the content property if it is not
a valid unquoted value, as defined by
https://developer.mozilla.org/en-US/docs/Web/CSS/content.

Test plan: jest
Merge branch 'master' of https://github.com/js-next/react-style into quote-content

Conflicts:
	lib/stylesToCSS.js
Merge pull request #77 from ianobermiller/quote-content

Quote the value of the content property