add %aunique to default path formats (#190)
add %aunique to default path formats (#190)

At this point, everything's in place except for the import workflow. Album
directory uniquification works if you import two identically-named albums and
then immediately execute "beet move". Currently, however, a couple of things
prevent this from working as expected during the normal import process:
- The tmpl_aunique method expects the item to be associated with an album in the
  database before it is called. This is currently not the case.
- With the multithreaded importer, it's possible for both duplicates to have
  their destinations calculated before either is added to the database. This
  means that neither will detect a duplicate at the time of the move/copy.
- Even if the above issue is resolved, it will be the case that only the second
  album to be imported will have a uniquifying string added. If we want to add
  the string to the old album, we'll need to explicitly move it.
Also, documentation on this feature is still forthcoming.

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