Add wrap_loaders() helper.

[carltongibson]

Currently one has to override the template loaders manually in order to add the partials loader. That's rubbish.

Much better would be a wrap_loaders() helper that could do this for you.

• Should be called from an AppConfig.ready()
• The template_partials app should provide a default AppConfig that does this.
  • People would just then add template_partials to INSTALLED_APPS and they'd be done.
• For completeness, an optional AppConfig that doesn't do this can be added, so that folks can opt-out.

(I'm happy to elaborate if needed.)