Add flatpak_image plugin

Valentin David wrote a plugin to produce Flatpak images which is currently part of the Freedesktop SDK project.

The same plugin can be used to produce Flatpak application bundles so I propose we move it to bst-external to make it more widely available.

The code itself carries no license so I applied the MIT license which the containing repository carries. We need to check with Valentin that this is correct before merging.

Alternately we could continue to keep the plugin inside the freedesktop-sdk repo and add a ./setup.py file so that users could install the plugin, but that would prevent us from using the existing documentation infrastructure in the bst-external repo.