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/optional_props/__snapshots__/jsfmt.spec.js.snap

@@ -22,7 +22,8 @@ y = o;
 // OK; we know that narrowing could not have happened
 o.foo = 0;
 // future widening is constrained
-function bar(config: { foo?: number }) {}
+function bar(config: { foo?: number }) {
+}
 bar({});
 bar({ foo: \"\" });
 "