Allow generating files for partials

[rmccue]

I'd like to be able to generate files for partials, allowing something like:

<partials>
        <entry>
            <name>contributing</name>
            <link>./CONTRIBUTING.md</link>
        </entry>
        <entry>
            <name>some-other-file</name>
            <link>./some-other-file.md</link>
        </entry>
</partials>

Combined with a template with:

<transformation
	writer="twig"
	query="partials"
	source="templates/my-template/partial.twig"
	artifact="partials/{{name}}.html"
/>

This is almost possible, but $project->partials is a Collection of strings, rather than a collection of Partial objects.

Expected Behavior

query="partials" should pass phpDocumentor\Partials\Partial objects as the node to the templates.

Actual Behavior

The query passes strings as the node. This then causes the dynamic artifact name of artifact="partials/{{name}}.html" to fail, as it attempts to find the string property of a string.

Steps to Reproduce the Problem

1. Add partial to configuration
2. Add transformation with query="partials" and dynamic artifact="partials/{{name}}.html"

---

This could be solved by moving the content generation step from Partials\ServiceProvider->register() into a new Partials\Partial->toString(), and storing the actual Partial objects in the collection.