Fix async switches (#110)

If any switches are async, the subsequent code will execute before the switches are finished. This PR moves all that code to a new function, and debounces the calls to onSwitch() so it only executes
once, after all the switches finish.

Fizes #72.

tests/setup.js

@@ -0,0 +1,6 @@
+var jsdomOptions = {
+  url: "https://example.org/",
+  runScripts: "dangerously"
+}
+
+require("jsdom-global")("", jsdomOptions)