Add the ability to extend the privacy config

Hi!

I wrote a SoundCloud shortcode for my blog, and thought it would be nice to have a privacy-sensitive version, so I looked at how the built-in shortcodes do it (Vimeo looked like a good candidate for inspiration). However I can’t add

[privacy]
  [privacy.soundcloud]
    simple = True

as Hugo doesn’t know what to do with it - it looks to me I would need to add SoundCloud support and recompile Hugo, which is less than ideal (although a pull request is a good idea which I’ll get to in the coming days).

Would you consider adding a mechanism to extend these privacy options?

Yes, that is a good idea. Please create a GitHub issue.

But to keep these separated from the buil-ins I suggest we just add a “custom” map (better name?), so you would do:

[privacy]
  [privacy.custom]
   [privacy.custom.soundcloud]
    simple = true

I’m not totally sure I got the TOML syntax correct …

That map[string]interface{} map would go in here somewhere: