Build-time configuration of index.html

*I'd like to gauge interest / comments for an easier way to customize index.html output.*

## Use-case

We have some dynamic values that we want to interpolate into the index.html file at build time. For example, static classes on the `` element and special meta attributes.

Ember already provides the `content-for` hook, but adding individual attributes to specific html elements is trickier.

## Possible solution

- Exposing the internal `configReplacePatterns` API, for example a `addReplacePattern` method or a key under config (environment.js), say `additionalReplacePatterns: [ … ]`
- Allowing custom 'slots' for use with `{{content-for 'my-slot'}}`.

## Reference code

https://github.com/ember-cli/ember-cli/blob/b24b73b388934796ca915ca665b48a27c857199b/lib/utilities/ember-app-utils.js#L169