Provide the ability to specify template loaders in a plugin manner

In GitLab by @todd-dembrey on Jan 15, 2018, 12:21

This would allow you to swap out to different render types (jinja2) or to provide standard wagtail tags as part of the core repo.

```
PATTERN_LIBRARY_TAGS = [
    # pattern_library.loader_tags.django,
    pattern_library.loader_tags.wagtail,
    pattern_library.loader_tags.jinja2,
    path_to.custom.loader_tags,
]
```