WIP: Adds partials to templates

Related to #974

Can be invoked by:

```
<?xml version="1.0" encoding="UTF-8" ?>
<phpdocumentor>
    <partials>
        <partial name="introduction" href="docs/introduction.md"></partial>
        <partial name="anotherIntroduction" type="markdown">
            <content><![CDATA[
                # Another introduction
                ...
            ]]></content>
        </partial>
    </partials>
    <translator>
        <locale>de</locale>
    </translator>
</phpdocumentor>
```
Paths are always relative to the project.