Wishlist:_mark_remotes_offline.mdwn

I have several remotes which are not always accessible. For example they can
be on hosts only accessible by LAN or on a portable hard drive which is not
plugged in. When running sync these remotes are checked as well, leading to
unnecessary error messages and possibly git-annex waiting for a few minutes
on each remote for a timeout.

In this situation it would be useful to mark some remotes as offline
(`git annex offline <remotename>`), so that git-annex would not even attempt
to contact them. Then, I could configure my system to automatically, for example,
mark a portable hard disk remote online when plugging it in, and offline when
unplugging it, and similarly marking remotes offline and online depending on
whether I have an internet connection or a connection to a specific network.