Sometimes the same repository can be accessed via two remotes. One remote
might be readonly, but fast (or not needing a password) while the other one
supports writing. The idea is to have a config setting that makes git-annex
use the latter remote for write operations when the user tells it to
operate on the former remote.

This is somewhat similar to `remote.<name>.annexUrl`, but that is for
different urls that access the same type of remote, eg http and ssh access
to a git repository.

This would also support --sameas remotes where a data store can be
accessed via two different types of special remotes.

The config could be something like `remote.<name>.annexWrite`.

Implementation would need to somehow handle cases where a command can both
read and write. Eg, `git-annex sync --content foo` with
remote.foo.annexWrite=bar should use foo for reads, but bar for writes.
So the implementation can't just replace the remote at parameter parse
time. Instead, it seems it would need to modify the Remote object for foo,
making write actions call to the Remote object for bar.
