command: start making global filter ctxt useful for interpolation
    
    The aim is to be able to specify, e.g., "{<filter>.in.<something>}"
    that would refer to some property? of the input object pertaining to
    given filter -- presumably the output files would refer to the input
    files, implicitly solving the time-dependency (thanks to real filter
    application ordering being already automagically solved through
    "backtracking"/postponing).  The property in question would likely
    be some kind of hash that would semi-reliably pair output(s) to input
    (goodbye overwrites loosing this pairing info).
    
    Signed-off-by: Jan Pokorný <jpokorny@redhat.com>