The `directory.html` template should be added in `locals`

In case someone wants to add extra content (e.g. an additional script ) in the existing template, it would be good to be able to do it also from inside template function (instead of copying/editing the default template). To achieve this, the default template should be added in `locals` object passed to template function.

The advantage of this is that on a later package upgrade, the template will be always up-to-date with the authors' version (plus the edits).

Locate the directory.html template and the template function that receives the locals object. Read how the default template is currently selected and determine how it can be exposed through locals without losing the package's default behavior. Done means callers can extend the default template from inside the template function while upgrades continue to provide the authors' current version.

Written by the indexing model from the issue text.