Improve CTM example

- Add meta and title tags, just to make it look like a valid HTML page (not sure of needed)
- Inject script using `appendChild` instead of a `document.write`
- Wrap text on 80 chars
- Put CTM test in a variable
- Swap the `core` class really early (before CSS is in) to avoid any FOUC
- Shorten the class replacement regex
- Remove unnecessary `type='text/css'` and `type=`text/javascript'`
- Add link to polyfill service documentation

We can discuss the loading method later (script tag vs injection). Interesting article on the subject: https://www.igvita.com/2014/05/20/script-injected-async-scripts-considered-harmful/

I was also tempted of adding this, which probably helps against having a failing CTM:

```html
<meta http-equiv="X-UA-Compatible" content="IE=Edge">
```