Current demo was impossible to debug index.js, now it is (#522)

Instead of pre-compiing the CJS bundle and then using that in the
demo (which makes `index.js` completely un-debuggable), we just
include `index.js` as source in the demo bundle. Now it's totally
debuggable.

Also remove the unnecessary `--` to pass parameters in `yarn start`
as this is not needed with yarn (and will later be passed to the
underlying script, whcih would break things).

README.md

@@ -177,10 +177,10 @@ Returns the `trap`.
 
 Read code in `docs/` and [see how it works](http://focus-trap.github.io/focus-trap/).
 
-Here's what happens in `default.js` (the "default behavior" demo):
+Here's generally what happens in `default.js` (the "default behavior" demo):
 
 ```js
-const { createFocusTrap } = require('../../dist/focus-trap');
+const { createFocusTrap } = require('../../index');
 
 const container = document.getElementById('default');