Set default WebVRConfig options in src/main

[cvan]

Instead of having to manually look at the README to see the default configuration options of WebVRConfig, it'd be nice if the defaults were defined there and overridden by anything set by the user on window.WebVRConfig.

[cvan]

It's worth noting that this will/could change if src/main starts exporting as a module (see issue #47).

Based on what many projects do, I'd recommend this:

module.exports = function (opts) {
  return new WebVRPolyfill(opts);
};