Add configuration to switch off the logic for localhost development

[borisschapira]

cloudinary:
  mock: true #default: false

With this parameter to true, the use of Cloudinary is disabled, as well as the creation of srcset and sizes. A normal image tag is generated.

This would make it easier to consult pages containing images that are not yet available on the Web.

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[nhoizey]

Looks like a duplicate of #20

[borisschapira]

Yep