Add --maximum-cost=N which prevents trying to access any remotes with a
larger cost. May as well add --minimum-cost too for completeness.

My use case: Want to git annex get --auto and pull from any of 3 usb
drives, but not from the network. --[[Joey]]

> Hmm, [[todo/to_and_from_multiple_remotes]] might be another way to do
> that. Put the 3 drives in a git remote group, or list the remotes on the
> fly.
> 
> There could still be benefit in avoiding high cost remotes. But, the cost
> numbers are only intended to create a local ordering, so making them part of a
> user interface is kind of weird. While 50 might be a high cost in one
> repository, in another repository it could be a fairly low cost. The user
> would need to examine all the costs to pick the cost they want; using
> remote names seems better UI. --[[Joey]]
