Inconsistency with jsdom.env

> ```jsdom.env``` is normally used to be the easiest access point to parse a website. However it doesn't load external scripts by default. By supplying jsdom.env with the proper ```features``` config it is possible to load external scripts, however, some bug in jsdom.env prevents that the referenced external script is loaded and processed. If, however, a src or scripts reference is supplied with the config, the referenced external scripts on that page are also executed.

I really like the jsdom.env api and the possibility to provide a ```features``` config object is really elegant, this inconsistency is annoying however.

Note: Inconsistency is not fixed with this PR (at the moment). I just want to get some quick comment if this should be properly supporter before I search further to fix this.
I think this whole `window.a` thing is unnecessarily confusing. Maybe:

- created: `window.isCreated = true`; assert others false
- script: `window.isCreatedBeforeScriptRuns = window.isCreated` + `window.scriptHasRun = true`
- loaded: assert both are true
- done: assert both are true