Archiving: Allow file name interpolation in target headline

Archiving: Allow file name interpolation in target headline

Matt Lundin writes:

    I had one more question/request concerning archives. Would
    it perhaps be possible to enable the file name
    substitution (i.e., "%s") after the double semi-colon, so
    that the file name can be used as a headline within the
    archive file? My ideal setup would be to have a single
    archive file for each year's work, with headlines named
    according to the original files of the archived subtrees.

    E.g.,

    ,----
    | * finances.org
    | ** archived item
    | * notes.org
    | ** archived item
    | ** another archived item
    `----

This commit does implement this request.