Embroider: Inject Default theme via <link> in index.html

[gossi]

At the moment, the contentFor() hook is used to inject the <link> element for the default theme. The idea is to remove all used hooks over time to opt out of classic build pipeline. To remove the <link> for the default theme, the idea is to tell consumers to inject a <link> tag into their index.html, which could look like so:

<link rel="stylesheet" href="{{theemoDefaultTheme}}">

and {{theemoDefaultTheme}} will be replaced with the proper theme url - see #39

Open Question

Is it possible to provide variables for index.html?