Add newline for empty blocks {} (#205)

I find it weird to put `{}` together. So I figured I would change it to have an empty line and ask for feedback :)

![image](https://cloud.githubusercontent.com/assets/197597/21739279/062adc80-d44c-11e6-8bb7-5e0768f3ddde.png)

Beware: most of the test cases are not representative of real code, you almost never define blocks with no content in practice.

tests/while/__snapshots__/jsfmt.spec.js.snap

@@ -61,7 +61,8 @@ class C {
     return new C();
   }
 }
-function blah() {}
+function blah() {
+}
 var node: ?C = new C();
 while (node) {
   var parent = node.m();