index-v2 'mirrors' fully settable from config
This lets mirrors: in config.yml be the same list-of-dicts format as it is in index-v2. This also includes a data format conversion to maintain the right format for the old, unchanging index v0 and v1 formats.
This does not define any new key names, I figure we can leave those open in this part of the code, then if they need to be enforced, they can be added to lint or something like that.
Edited by Hans-Christoph Steiner