Initial CSS support (#1636)

I wanted to see how hard it would be to add support for CSS inside of prettier. Turns out, it's not that hard. I spent a few hours printing post-css values and getting all the stylefmt unit tests to not throw.

bin/prettier.js

@@ -90,7 +90,12 @@ function getParserOption() {
     return "flow";
   }
 
-  if (value === "flow" || value === "babylon" || value === "typescript") {
+  if (
+    value === "flow" ||
+    value === "babylon" ||
+    value === "typescript" ||
+    value === "postcss"
+  ) {
     return value;
   }