As suggested during the first Gitify BoF during DebConf13: Adding a way to have on-demand dropping of content in a given remote would allow a user to quickly free up disk space on demand while still heeding numcopies etc.

> [[done]] as --size-limit. This does affect dropping, so eg
> `git-annex drom --from foo --size-limit=10gb` frees up 10gb from remote
> foo. --[[Joey]] 
