Config default image

Image field formatter allowing to set a default image deployable through config management. It stores a file path into config, instead of a content uuid.

This addresses the problem that the core default image feature uses a file content. If you use config management (drush cex/cim...) you will deploy the config but not the content, so the default image wouldn't work in production.
A blog post about this problem: https://www.58bits.com/blog/2016/04/13/synchronizing-default-images-ima…