[settings] add option to define global path prefix for images URLs

[nhoizey]

Such a setting:

cloudinary:
  …
  images_root: "/media/"

Would allow users to write this:

{% cloudinary logos/colourful.jpg %}

And the generated URL would include <images_root> between the site domain and the image path:
http://res.cloudinary.com/<cloud_name>/image/fetch/c_limit,w_320,q_auto,f_auto/https://<your-domain>/<images_root>/logos/colourful.jpg