closes #44

Checklist:

• add HotColdBackend
• add --repo-hot option
• add checks comparing hot/cold file lists/sizes to check for snapshots, index and pack files
• add check comparing hot/cold files for key files (when reading keys)
• add is_hot attribute to config file in hot repo
• let init init a hot/cold repo
• add option to generate hot repo from existing repo
• show hot repository in repoinfo