When a namespace is specified, register partials in that namespace

A namespace is specified for many reasons, most involving the idea of being able to dynamically look up templates at runtime. There are cases where it becomes important to also have access to partials using the same mechanism that’s used for templates.

This PR modifies the `registerPartial()` call to additionally add a reference to the partial in the provided namespace. If no namespace is provided then the `registerPartial()` call is unmodified.