Add support for multiple media libraries

[aminimalanimal]

I would like to use Netlify CMS’s git-based media library for icons and Cloudinary for larger images and videos. If there is a way to set image management on a per field basis, I haven't been able to find it yet.

The image widget currently supports media_library configuration to occur within the field, but seemingly not selection.

What if setting media_library to null/false/local/netlify allowed the built-in media manager to take over?

  label: Icon
  name: icon
  widget: image
  media_folder: /assets/icons/
  media_library: netlify

This thought could go further, such as wanting to use multiple third-party services at once. Perhaps it should be possible to globally register multiple media libraries by name and choose which one is used per field.

[tillschweneker]

This would be great as it allows to use Netlify CMS even better as a page builder.