more metadata from docbook
    
    anerist will now extract 'taxonomy' and 'tags' metadata from docbook
    sources.  This requires a somewhat specific method for presenting the
    metadata:
    
            <articleinfo>
                    <subjectset>
                            <subject>
                                    <subjectterm>Scope/Topic</subjectterm>
                            </subject>
                    </subjectset>
                    <keywordset>
                            <keyword>some word</keyword>
                            <keyword>appropriate</keyword>
                    </keywordset>
                    ....
    
    All values of `keyword` will be added to the document's metadata.  Only
    the first value of subjectterm will be applied, because there is no
    implied ordering in the stack, so the heirarchy will be done at the end
    of the process to minimize disruption of the taxonomic heirarchy.