`git annex export` normally exports all files in the specified tree,
which is generally what the user wants.
But, in some situations, the user may want to export a subset of files,
in a way that can be well expressed by a preferred content expression.

For example, they may want to export .mp3 files but not the .wav
files used to produce those.

Or, export podcasts, but not ones in a "old" directory that have already
been listened to.

It seems doable to make `git annex export` honor whatever
preferred content settings have been configured for the remote.
(And `git annex sync --content` too.)
